With a lot of cycling gear it is a case of you get what you pay for. Sometimes by trying to save money you end up having to replace items more often.
As others have said more expensive kit i.e shorts will have better padding and being uncomfortable in those regions when on a ride is not a nice thing for a bloke.
The thing to do is keep an eye on all the big retailers for their sales and look for the better quality stuff when it has large reductions on it.
What is the threshold between cheap and expensive? Like what would you expect to spend on a decent long lasting pair of shorts?