Voices.com is much like Voice123 where as the talent pool can vary greatly in skill level. Voicebank can post an ad for you to the top talent in the country through their respective agencies.
As for the cost of a :30 TV, you would have to answer a couple questions first:
What is the market size?
and
How long will the spot run?
The larger the market and a longer run will cost substantially more, than say a local small market for a few weeks.
You are looking at anywhere from $100 to five digits or more, if you want a celebrity.