Dont forget there are many features of a sump not found in canisters. Like maintaining water level in the aquarium, and providing access without having to get into your tank for things like water changes, adding chems, taking water samples, etc.
Cost depends. If your DIY'ing it you might have to figure in the cost of re-doing your plumbing 3 times and eating the cost of shipping for 3 pumps you sent back because you didnt research it enough:irked: .... lol
Even though my sump is extremely simplistic I've probably spent the cost of an Eheim Pro3 plus another canister on top of that. Dont forget the cost of the overflows too, for my tank that already put the cost at over $200. Add 4 bulkheads and misc parts for the durso standpipes and that another $60+ and we still havent got below the tank yet. Ended up not being nearly as cheap as I thought, actually ridiculously expensive. next time I'll be trying the diy pvc overflows.