Don't forget that the Ruger 30 has had issues with firing pin breakages. Firing pin replacement on a M1A is can be done relatively easily. A bolt disassembly tool, which makes the task very easy, is available from many vendors.
If you break the firing pin in a Mini-30, you very well may have to ship the whole rifle back to Ruger to have it fixed.
Is there a cheaper semiauto alternative to the M1A?
Not really. Maybe the FAL or an AR10 but they are getting expensive too. The M1A definitely holds it value.