Even though I'm a relative 'noob' to the scene, i've been lurking in the background for a fair while and can let you know how much foam I have used over the few basic first-steps builds I have done.
The foam I have been using is not the best in the world - a 1.8m x 0.8m (70" x 31.5") roll of 8mm (0.3") campermat - but it's been working quite nicely for the builds I have made....
I have so far made....
1 roll :
IM Mk 7 Faceplate
IM Mk 8 Full Helmet
IM Mk 39 Chest
and please bear in mind that there is a LOT of scrap generated when you're working with foam.
However, as these rolls cost me something like £5-6 ($9-10 US) each... I don't think that's too much of a problem

And yes, when I got them, I picked them up from one of the 'town centre' camping shops, so got utterly ripped off. I've seen them for sale for half that amount.
Broadly speaking - I'd suggest that you'd need between 3 and 5 rolls of foam for a full suit of IM-type armour, depending on the complexity and how many times you screw up the cut / the shape / the gluing phase of construction.
There are pro's and con's to both paper/card pep and foam pep... but if you are 'sneaky' about it, you can use both in the build.
First off, make the foam build... and then to every 'part' of the model that needs extra detail, or harder edges, or sharper lines or whatnot, build that particular piece in paper/card and glue it onto the foam. Then it would be a case of hiding the card cut-edge in the 'sanding/smoothing/bondo/resin/fibreglass/rotocast-plastic' phase of construction.
As the saying goes - there's plenty of ways to do this - experiment