The prices for boosting can vary dramatically. The biggest determining factor of price is the desired division you are looking to get boosted to. Think about it, a boost from Bronze 5 to Bronze 1 is going to cost a lot less than Diamond 5 to Diamond 1. The higher you want to go up the tier list, the most skilled booster your will require.
It's just like in game, there are hundreds of thousands (if not millions) of people in Bronze league, yet there are only a small handful of players in Challenger tier. Combine this with simple economics, and if you want to reach Challenger, then you'll have to pay a much higher price.
However, that being said, most boosts only cost tens of dollars and range from $10 all the way into the thousands. If you're looking to increase your division by a few ranks, then you won't be looking at much.
LoL Placement Boost
LoL Division Boost