Javert makes next to nothing. His rank of "inspecteur" (also known in some locales as "agent" or "garde-de-ville") makes him an assistant to the officially appointed commissaire de police (CP). CPs were nominated through the Ministry of the Interior in Paris and were in some ways a political as well as security position - there were purges after every revolution so that men loyal to the new regime could get these government positions, partly to reward supporters but partly to maintain surveillance that is friendly to the regime. Agents were generally hired by the mayor of the town, who was also a Paris appointee, but that doesn't mean the mayor and the CP got along.
An agent might make as little as 150 francs a year (in fairness, a CP might make only 300 francs a year and have duties that make it difficult for him to hold down another job - frequently, the men in these low-paying districts were retired from the civil service or army and had a pension in addition to their salary as a CP). Agents frequently held other jobs in addition to their position as a police agent. This might include contract work: one agent in Poitiers earned an extra 100 francs a year for taking on responsibility for inspecting the city's butcher shops.
Agents were not required to have uniforms. They also had no real authority - they could not make arrests legally but were expected to alert the CP, who had that legal authority. They also had no duties directed by the state - duties of an agent varied by town, while a CP's duties were set by the Ministry of the Interior.
This is all directly from John Merriman, Police Stories: Building the French State 1815-1851, Oxford University Press, 2006. This book focuses on how policing was structured in the provinces and what it meant for the men hired to do it.
Javert's situation does not fit with what Merriman is telling me. If Javert is merely an inspector, he shouldn't have been sent anywhere by Paris. He is of lower class than is typically selected for a CP, but about right for an inspector/agent. M-sur-M has a bit over 4100 people at this period, which is not quite large enough to be required to have a CP, in which case, a town would frequently hire an agent themselves, but again, Javert was sent from Paris, and M-sur-M, with the port, is plausibly one of the smaller towns that were required to take a CP in exception to the 5000 population rule. He acts as if he has power of arrest. And Valjean doesn't have the authority to fire him - what Javert asks him to do is to go to the proper authority to demand Javert's dismissal.
So I think, despite Hugo's use of "inspecteur", Javert is actually a CP. CP's salaries were determined by Paris within six salary bands based on the size of the city being policed. M-sur-M would be in the lowest of those bands. The municipality was expected to pay the minimum, but they could augment this salary as they chose, since they were the ones paying the whole salary. In this case, I would guess that the 600 francs a year that the CP in Saint-Gaudens got during the Restoration is a reasonable estimate for M-sur-M, moreso than the criminally low 300 francs a year the poor guy in Neufchâteau was getting at that period. M-sur-M is between the sizes of Neufchâteau and Saint-Gaudens in the 1821 census, but Valjean is the mayor, so the higher 600 figure seems reasonable to me.
The trouble here is that there's a lot of variation. The guy in Saint-Gaudens was getting 600; the guy in Brioude was getting 800, and Brioude had a heck of a time keeping anyone on such low wages. But other towns weren't having the turnover Brioude kept having. The difference may well be the difference in cost of living in the Haute-Garonne vs. the Haute-Loire.
We don't have enough information on actual cost of living in M-sur-M, but we can at least suggest that if Javert is a CP rather than an inspector, as seems likely, Valjean will ensure that his salary is just high enough that he can afford rent on a bachelor apartment and not worry about having enough to eat. Javert also probably had a decent budget for office expenses, which not all towns provided (they had to provide a budget, but it didn't have to be a decent one). If your town didn't want to give you much in terms of office expenses, you'd pay those out of pocket. This includes your basic office supplies but also your travel expenses and all that sort of thing. So Javert likely is keeping his whole salary, and he has no dependents, so he's probably in pretty good shape at around 600 francs a year.
But only because he has no dependents and M-sur-M is one of few small towns with a CP that would come up with the money to make him comfortable (because Valjean would pay it from his own pocket if necessary). In general, we're talking poverty level wages, but for men of education. Yet this is where you're paying your dues; once promoted to Paris, Javert would enjoy a salary of between 3,000 and 6,000 francs a year depending on rank within the Paris system. 3,000 francs a year is nothing to sneeze at from a working class perspective. At that point, he can easily afford to support a family, have a servant, and generally have a perfectly decent life. White collar working class - clerks and that sort of thing. Steadier than keeping a shop, since salary is guaranteed year in, year out. And certainly making bank compared to the poor guy in Neufchâteau.
So that was long, but Hugo made it a bit complicated with his terminology. You've got answers for both options.
(there's also another level down, the "mouchard" or police spy/informant/infiltrator. These guys aren't salaried but are paid for information or by the job and come from the lowest classes. They don't have to be literate. They don't have to be anything except willing. Agents are drawn from what you might call the class above these - the people capable of holding down a job, since they're generally holding one down in addition to serving as an agent. The mouchards, generally not.)
What kind of literature and what kind of life is the same question. - Tom Stoppard