in essence it is a case of you get what you pay for. by not going with the pre-made system you will have the benefits of making the sump any size that you can find a container to fit under the tank, you will be able to easily design in a refugium later, you will be able to more easily lay out your plumbing because inlets to the sump will not be factory determined... also you will not be paying for their plexiglass, their weldon, their bioballs... all things that you really do not need to make an effective sump. if you took the base price of their product, and subtracted the value of the plexi, the value of the bioballs, and the value of the time for someone to assemble it, you can see the true value of what is left, namely the skimmer and the overflow box. i do not know the costs associated with building that exact unit, but lets say it is $100.00 for the sake of easy math. that means you are essentially paying for a water holding bin, a skimmer, a return pump, and an overflow for around $140.00 give or take a bit. now if you were to compare shop and find your own skimmer, return pump, and overflow for around 140, you will be able to quickly see the quality of equipment you are working with.
incidentally, most of those that say includes pump, mean the pump for the skimmer, not the pump for the return, so you might want to look more into that in detail.
so basically you do have an option. you can spend your 250 or so getting a good solid skimmer, a properly sized sump, a good flow overflow, and a good pump for your return (one of the most important places to have quality by the way) or you can spend the same ammount of money, or a little less, to pay someone else to do it for you, which means labor, materials, and profit, which from my experience and basic reasoning means inferior product.
and that my friend is in essence the theory behind diy, you are either making something nobody else will, you are making it better than anyone else will, or you are making it cheaper than anyone else will. if you seek to satisfy one of these situations, or in part all of them, make it yerself!