I just priced up similar at fhbrundle in birmingham and it came to £350 inc vat and delivery. But bear in mind that I think they only sell in 7.5mtr lengths so I was ordering quite a bit more steel than you have listed.

Steel Sections from stock to meet any fabrication specifications - F H Brundle

Not to sound to nuts, but have you considered filling the beams with some, erm, epoxy...?

You could also save a bit of money by dropping down to 80x40 & 40x40 instead of 100x50 & 50x50, just make sure you can still mount your rails.