You can spend according to your needs.
You can use Google adwords which is the most effective but expensive.
There are some cheaper ones such as Yahoo and AdCebter of MS.
It depends on how much budget you are willing to put in. If you want an aggreassive campaign for top keywords, you would need a bigger budget.
I usually go with the long tail keywords that are cheaper. Research first on what keywords you want to opt for. Don't just go bidding on top keywords as you learned that they are the ones that makes the most clicks, this would empty your pockets before you see some results especially if you are a noob at this.
Also try using smaller PPC engines first so as to test and experiment a bit - they are usually cheaper.
Pay per Clicking will cost you alot and it depends that how much you wanna spend to promote your site but i will suggest you to go for Search Engine Optimization.