If only they would actually give us a way to spend money... Who wants to pay $100 per Prestige building to speed it up? Not many people... Make the price reasonable enough that a lot of player would consider it.
How about an economical way to buy level of skins/units? Right now you can buy a crate in the store for $5 for 4-12 frags where it takes 900 frags to get the average skin/unit to level 10 or 11. Once again, that is $1000s of dollars to max out a single unit. BUT those crates have random frags from several units so you also have to pay for all of those other units. Who wants to pay that much for the one or two things they want along with tons of crap? I'm sure if they allowed it and priced it right, there would by tons of players willing to spend a bit to max out jets, snipers, buses, etc and they would make a ton of money.
Until they make it so that one can spend a reasonable amount of money some something that I actually want. I will continue to paly for free. But I would really consider spending a reasonable amount of money to reduce the amount of time that I have to spend grinding for frags that I want!