You wont get a firm answer on the amount you can expect to spend as mentioned above. However what I can tell you from experience is you need to do some serious keyword research before investing any money. Find high search volume keywords with very little competition. Also remember that the more you can target your advertising, the more effective it'll be. Using both of these will help you save quite a bit of money.
If you are in the startup phase I'd also look more towards homegrown alternatives before jumping straight to PPC. Stuff like offline marketing, viral marketing, social media marketing, etc. But I dont know your product or service so I can't promise you that that is the right solution either.