They were recently going for $75 for #1 grade blocks. This was a sale price, normally $75 for #2 grade and more for #1.
I wouldn't touch it without having an geo tech engineer look at it. I'm not too familiar with brick and mortar retaining walls but the material alone would cost a lot I would imagine. I personally would do a backcut and then do a butress fill with a gravel subdrain every 10-15 ft in elevation to rid off any seepage at the contact line. Not sure if that is a common practice in your neck of the woods but I have done many butress slopes here in Cali and have never seen one fail.