I dont think the one I am building will ever pay for itself. I just really like to build things and I really get the satisfaction of making something and then seeing it work like I wanted. I do a little wood work, but I really like playing around with acrylic too. I am to cheap to buy my skimmers, reactors, or tanks for that matter so I justify this to myself in every time I want to try and build a new toy, I have to right tool to make it.
I figure I will have $1500 in my machine, but it is not one made of wood and it uses real linear bearings and rails so it should last forever and be fairly accurate with the cuts. I have been doing a lot or research in the past couple of weeks and all the machines have the same basic concept. I am going to use steel for the frame and gantry to mount the rails on (scrap steel is cheap) basically so I dont get any flex. So far the rails and bearings where the high dollar parts. You can get a complete 3 axis kit (electronics) for $400 and using acme leadscrews seems like the best cheap way to go for around $200 total. So for my $1500, I am going to build something that will do what ever I want and maybe just upgrade the electronics if I want something faster in the future and not have to build another complete machine.
BTW, aluminum is a lot more than steel, but it is real easy to drill cut and it is a lot lighter. Just my 2 cents.