The base components that make up your voice can only be altered so many times. I would say a single talented voice actor can do maybe 3-4 voices before it becomes very noticeable that a single person is doing all of the voices.

What CCH said is true. Even really talented, experienced actors can usually only pull off about 3-5 characters and then it starts to all sound the same. I would record yourself for 2-3 characters and then hire others to fill in the gaps and provide more variety.

You'd be surprised at the range of prices voice actors, especially young(er) ones, will be willing to accept. There's also websites where actors can bid on working for your project (voices123.com for example). If you're a student or have a college near you - check out local drama students that might have the ability to record themselves. Of course, the old saying "you get what you paid for" usually rings true. But with some careful research, you can find someone talented, who's just starting out and willing to work for less.

The other idea is to limit the amount of voice over in your game. Instead of having VO for an entire paragraph, maybe just the firt sentence and have the rest be text? Also you could consider having only critical characters have VO.