Oh empty tanks are not an issue i have empty 10g, 20g,29g,55g, 20g long(with a bulkhead in it). Its more the overflows they can get realy expensive compared to a HoB (which is stupid) . Pumps also get pricey. I was going to do a DiY pvc overflow at one point .... but there were so many horror storys of failed pumps and flooding :-/. makes me kind of iffy. And again I would hafto build a wooden stand as a sump will not fit under my 125g with the cinderblock stand.
I am sure I will get the itch to do one again at some point just a question of when I will pull the trigger.