In the early/mid 80's, when I was an operator, they were about 3500- roughly. You made you money back in a short time. Now they are much higher, and it's a much longer ROI. I'd guess in the 70's, roughly half of this.
I read a breakdown a long time ago, can't remember for sure, but I think it was around $500 in the late 60's-early 70's. The SS machines after '77 were $1,500-$1,700. But it was the Steve Ritchie-designed Williams games that first pushed the price up to over $2,000 (Black Knight), and later, $3,500+ (High Speed).
That is, I think.