While it can be quick and dirty to build a screen yourself, I found that it took me about 8 hours to do it completely. It also ended up running me about $200 or so in materials. I used poplar for the wood (1x3's) and I used baseboards wrapped in velvet for the frame. It all turned out very nice, but it was a lot of work to put it all together, and the price to get quality raw materials was a bit more than I had hoped for.
There are certainly ways to do it for less, but I really want there to be a VERY deep black border around the edges and I wanted the beveled edges for things. So, there was a cost for all of that. I also wanted a nice stable frame, so the poplar wood instead of pine and then I used steel right angle brackets and decent hardware at all connection points for best strength/quality I could deliver.
Your sizing is good. I like that you are going a bit larger so that you can trim it up with the border you want to use.
It certainly is a good project for anyone who is handy, or just wants to be handy, to work on as it isn't that difficult overall. Make sure you get some help when it comes time to wrap the screen. You want that pulled tight and to use a lot of staples!
At the end, I didn't think the screen looked one bit better than a Silver Ticket screen, and I could never break it down to move it.
$250...