you could get this welded and it would be just as s trong....and for (i would think ) a lot less than 150 each...most backyard welders can make these things...and a mahine shop can make them very quickly...to bend that corner and cut the peice would take about 30 seconds each after set up..the welding of the pipe only a few minutes....look around your neighborhood...if you are a doityourselfer...i am sure you know a few more whom could make these for you

I think you'd be alright to let him weld them, just be sure he welds it both inside and out. A weld is almost as strong as the metal, but if the metal on each side of the weld isn't preheated it creates stress along side the weld and can crack in that area. Make sure the deck is fastened securely to the frame, better yet put in some diagonal bracing on the frame to counteract wave action making the frames get out of square. I don't know what he'll charge, but I think $115 is a little out of line on the link you provided. They have a metal brake to bend it and a punch press to make the bolt holes and quite possibly a robot welder to weld the pipe to it.
Hope this helps,
Mike