Well I guess everything it's used for now. It could be a foundation for another proposal where departments start with lesser equipment/supplies and will beed to spend their budget to order more but this isn't part of this suggestion and I'm not sure how I personally feel about that one however it does lay those foundations for later on if we wish.
I'm sure there's things that will come up that need to be tweaked to make it "balanced" because at the moment costs of goods/services and the way in which people can earn money is arbitrary and not really designed with persistence in mind but I'm sure we can address those as and when. We'd need to have it exist first to be able to gauge and produce a realistic supply and demand model. I don't know how much to charge ciggies because I don't know what the spending power would be for characters with this change. It will still be arbitary figures as we have no market data because we've never had a market like this.
Regarding allowances - I imagine it'd be a staggard system based on role and race, with human captains having large allowances and vaurca assistants having low and all inbetween. In fact thinking about it, it sounds even more fun. Now when I do a job for a scientist as an assistant and they tip me 100 credits, I'll actually be happy to have it because of my allowance unlike now where it's essentially meaningless unless I RP it to be so, which I of course can, but I can RP anything without mechanics to be honest. I like the idea that being tipped or given a bonus would have an effect in game. Obviously command can hand out bigger allowances to whoever, their friend maybe if they're corrupt, but this might net them a CCIA visit assuming of course the accounts can be monitored. Silly things like a captain blowing the station budget on corgis would be a whitelist issue and thus we could just roll back this "shitspending" and discipline the player.