You can use Google adwords tool to get a general idea of what keywords cost, and you can also go into adsense and set up your ads so every single ad has a different tracking channel. For example, if you have a 3 page site with 3 ads on each page, you can make 9 seperate channels and 9 seperate ads. By doing this, and staying on top of your adsense stats, you can get a general idea of what pages and what ad locations are performing best.