Cross bolting a block that's not meant to be cross bolted is asking for trouble. also with the cross bolts, your adding clamping on the farthest point away from the crank. yes its better than nothing but not ideal.

the girdle adds clamping right on top of the main caps.

in terms of additional cost, whether you use steel caps or aluminum caps or even cast iron caps from another engine you will always need to line bore the block. all aftermarket caps are made undersize to ensure this happens. the only additional feature the bcr girdle needs is a flat oil pan rail, in most cases when guys mock them up, they don't need more than a few thousandths taken off to make sure everything is flat. this should only costs between $100-200 more at the machine shop. still a long long way away from the cost of an aftermarket block.