New 6 Color Auto that takes a 23x31 frame with a 16x18 print area in the market right now?
Cheapest will be pneumatic heads (obviously). Anatol Horizon, M&R Diamondback, and Workhorse Cutlass should be your cheapest options. Call and get pricing. Then compare features, reliability, service etc and pick your favorite. Begin negotiating.
Not sure your needs, etc. But I'd also get pricing for AC heads and 8/10 configurations.