I suspect £500 would probably get the materials for a basic one, metal pillers (in a reasonable amount of concrete to hold it securely), and wooden frame for the roofing sheets to be fastened to, depending on where you get them from/what you use.
I suspect the problem would be making sure it'll be rigid when it's windy, as they have a large area to catch the wind, so it'll need to be fairly solidly built (especially if it's in the open with nothing to act as a wind break).
The way they were/are built here (70's houses), is one side is supported by a wall, the other by metal pillers with additional supports in the middle, and a wooden frame the sheeting lays on (although this is for semi detached car ports, where the wall is actaully also the boundary wall).
My dad took ours down many years ago because basically it wasn't much cop, due to it's location the wind was not kind to it
I'm not sure if you'd need planning permission to put one up?
I suspect if you have a look round there is a good chance there'll be a company who will either be able to supply a "kit" for making one (all the parts and plans), or even just proven plans for one.
Nexus, I suspect they've gone out of favour because they take up room that could better be put towards another house (as far as the builders are concerned
) and don't give you the relative security/generally useful space a proper garage does.
*Allowing for the cars to be about 5 meters long, by 2 wide, and the sheets as the standard 3x0.6 size, and of the cheapy type (at about £7 a sheet), you would need about 15 sheets (although with a fair bit of wastage).
