Actually OP don't look for a cheap site to convert your btc to ethereum but instead the one you need to find is a reliable trusted site where you can exchange with at ease and that is binance and OKEX. these two site is much recommended since they are the one who stays strong for more longer years so it's good to go with them since if you trust easily those new ones for sure there's higher risk for you to lose your alt for those scam sites around.