Yeah, theoretically, the cost difference could be as little as $500, for a SOLID roller. Hydraulic roller is the most expensive option.
Depending on who your builder is the cost of the parts will likely be much more than the price I've listed for two reasons. #1 you'll be paying top retail price, even if you think your getting a discount, the cost will be factored in some other area. #2 it's unlikely he will build your big horse power Cobra motor with "basic" parts. All though you could go with a local builder who will indeed build it to your budget, caveat emptor.
Here comes the kicker:
The price of the parts will rise (like the ramp on a roller cam) considerably faster for quality roller parts than it will for quality flat tappet parts.
Premium parts would be the logical standard for a good quality builder if he offers a warranty. You could save even more if you went with some non-brand name (solid or roller) lifters compared to a well known brand name lifter. But "no name" lifters are notorious for being inferior, this is one area where I will not scrimp! I wonder how many flat tappet cams have been lost due to inferior foreign lifters and the cam manufacturer is the one who takes the heat???
The "savings" could buy you a nice Armando road racing
oil pan. Or a nice set of March pulleys, etc. etc.