We've had this happen on two different occasions with two different showers.
The first was in our new home about 2 months after the home was built. The builder forgot to put supports under the pre-fab base. They came in, cut some out, added supports and then had a fiberglass guy come in and redo the base. Worked great after that. I dunno what the cost was as the builder covered it.
Most recent was in our current home. Shower had a history of leaking. Was a combination of a cheap pre-fab that was used that did not have enough support, and a poor seal on the surround.
I did not feel I had the skills necessary to do a poured & tiled floor, so I shopped around and found a pre-fab base that was very thuroughly supported on all areas of the base. Installed it and it has been very solid.
I think it is a 'you get what you pay for' deal. Some builders go cheap..and others go with quality. Unfortunately the home owner dosen't usually know until they have a failure.
With yours, even if the crack is sealed, since there is probably a lack of support under the base it will probably crack again someplace else.
What type of surround does your shower have? We had tile and ended up finding a matching tile and just removed the first two rows to pull out the bad base and slap in a new one...retile..and wala.