Well, only you can decide what you mean by "cruel" and why you believe that applies to a surrogate situation.

The majority of surrogates I know about are either relatives of the couple trying to have a child, or they are professionals who have been a surrogate for several couples before.

In the least legally-tricky way to do it, sperm and eggs are collected from the couple trying to have a baby. The sperm fertilize the eggs, and the resulting embryos are implanted into the surrogate. She then has no biological connection to the child.

There really are women who view being pregnant with another couple's baby as a job.

In my personal opinion, it is a perfectly fine way to enlarge a family. Just employ all of the legal safeguards - contracts, using your own eggs and sperm if possible (even though using her eggs would be cheaper), etc.

Although, you definately want to first learn WHY you miscarried. If it's a problem with your uterus or something else in your own system, using a surrogate might solve the problem. But if it's a problem with egg quality, or a genetic issue in the embryo itself, then using a surrogate wouldn't help.