Bitcoin would be a poor choice for such a system because it doesn't really have a concept of account balances that are present in the blocks. You'll likely also need some central authority to decide who is part of "everyone". Otherwise, someone can create a million accounts and then pool all the money.

Also, Bitcoin is fundamentally secured against double spends by mining, and that makes it a poor choice for any currency that can't pay sizable enough rewards to produce real security.

You'd probably find it much easier using some scheme based around accounts with balances rather than unspent transaction outputs.

If you want to implement crediting with Bitcoin, you'd need to create your own implementation of a bank. You would probably create it on a basis of an eWallet with some additional accounting code thrown in. It shouldn't be too costly to create, but it could get a bit expensive when you'll be trying to secure it against abuse. Recently there have been too many cases of Bitcoin thefts.

However, you could get pretty much the functionality you describe (letting people spend up to X amount of Bitcoins a month plus whatever they get), by giving people Bitcoin addresses to use and just topping them up with a pre-set amount of Bitcoins. This way they could spend precisely the amount of Bitcoins you gave them plus whatever else they receive, and you could check their balance at the end of the month through a Block Explorer. It would be like a pre-paid debit card in this regard. This solution might not be precisely what you have in mind, but it appears to be close. This solution would be quite cheap, as you'd only need to have some accounting program running once a month on top of bitcoind more or less. This would mean a lot less potential chances of your Bitcoins getting stolen.

All in all, it's not too expensive to create financial Bitcoin applications, but a lot of your cost could go into securing your users finances.