Slev:
You seemed to answer your own question…but it’s a good question:
Building your own keg box is completely up to the builder.
The cost savings are probably more than $50, but it is also the fact that the keg box can be sized to hold more than just a keg. Mine holds a half barrel and up to six Franzia boxes of wine as well as a few bottles of champagne or wine. It can also be easily modified to hold two full kegs.
The other advantage is that the tap tower can be mounted into the bar top which looks nice. This part of the project is not just about saving money, in fact you forgot to add the brass drain tray, another $70.
Here’s a photo of my keg box loaded with a quarter barrel and wine.
http://www.barplans.info/gallery/displayimage.php?album=11&pos=15
Keg Box Tower, Brass Drain & Bar Top: http://www.barplans.info/gallery/displayimage.php?pos=-585
The whole Keg Box, door closed: http://www.barplans.info/gallery/displayimage.php?album=11&pos=14
Satisfaction is one other reason…it’s just cool to build your own keg box.
If you are so inclined, go for it, it works great. If it sounds like too much hassle, buy a kegerator and work it into the plans, many have done so.
Store bought kegerators are just fine, again, it’s your choice.