I'd make it how it is in many other communities; where gangs can ask for a base, but depending on what they have it may be added. A gang can design a base and ask for it to be put in a certain area (one cleared by an admin of course). An admin would then look over the gang, and decide by looking at (for example): member count, overall reputation, etc, etc. There would be an initial fee of however much (Eg. 300k or something) if it was to be added, but that would be the end of it when it comes to payments. The cost would be higher if the base that is designed includes a helipad, garage, private shop, etc. If that base is not used much or the gang disbands, then that base can be removed or put up as a choice for other gangs if they would like to buy it or design their own. And so on would the system go.
I'd even make an application and those accepted can ask for a base, making it possible for the smaller less-known gangs to have a chance.