Corus INSTALL+TM, welded steel tube (also known as gas list and previously specified to BS 1387) suitable for general purpose
conveyance and pressure applications.
Thats the posh name for it anyway

Now, pipe is sized by its nominal inside I.D. and this is what you ask for, but you also need to specify "medium" or "heavy" and this refers to the wall thickness.
Andy (manky monkey) uses 1" heavy pipe on his frames. Thats 1" I.D. with a wall thickness of 4mm and an O.D. of around 33.5mm.
For my
last current one, i opted to go smaller at 3/4" heavy. Thats 3/4" I.D. 3.2mm wall and O.D. of 26.9mm.
That`ll take care of the majority of the frame, the main top centre rail needs to be bigger at around 1 1/2" or so.
Its really up to you which you use, personaly i feel the 3/4" is fine but that is the smallest you wanna go

Price wise

its been a while since i bought any and the price on steel fluctuates a lot. £60 should be in the ballpark, but dont quote me on that!