Most hashing contracts (not all but most) are scams of some sort. If they are legitimate, once you do the math they won't be cheap and there will not be any assurance of return. In this business, like most businesses, you can't get something for nothing. If a hashing contract is legitimate you will have to pay them an amount such that they can bank on a return on their machines - and you take all the uncertainty of bitcoin price falling or difficulty rising.

Nicehash is mostly legitimate (not strictly a hashing contract but a rental of hashpower), and once you run the numbers it works out that you will pay very close to your expected return.