Not going to cost that much to be honest, although it does require me to make the chest armour and the M3 tactical helmet, which is in progress (I'll post some pics when I've got something worth seeing).
The helmet and armour are being made from craft foam (20 quid off ebay inc p&p) with some web belts and bits of trim added to them, then there is a drop leg holster attached to the armour (we don't see the pistol much in the movie, so I guess anything which fits my drop-leg holster goes).
The MI uniform clothing is pretty basic, it's just a grey short zip-fronted jacket (8 quid off ebay inc p&p) with some black trim added to the collar and sides and some elbow patches in tan added to it, then three unit patches (which I got off ebay for 13 quid inc p&p) sewn onto it and black epaulettes. Under that is a black t-shirt and a black neck gorget. The pants are just plain grey ones with some black trim added along the inside leg (not got those yet, but I expect they'll be about ten quid).
The original movie prop boots were in fact just trainers modified with the addition of higher ankle support plus a few velcro straps added, so some lightweight modern US combat boots can easily be modified to look like those. The Mobile Infantry's rifle is the (fictional) bullpup configuration Morita Assault Rifle, which (depending on which soldier was wielding it), is either a full-length Mk1 rifle with an underslung pump action, or a shorter Mk1 carbine version, or a scoped sniper version of the standard rifle, all of which are seen in the movie. Since the Morita is (like the movie itself) a bit of a parody of the rather politically incorrect fascist overtones in the Heinlein novel, it's somewhat comedically over-large (also because it was in reality a fibreglass shell which had a Ruger Mini 14 concealed inside it for the shots in the movie where the Morita was required to actually fire).
In practical terms, I would like the load out to actually be usable, so I'm going to make the less stupidly big carbine version of the Morita (which also means I don't have to add the grenade launcher). Looking about for a suitable rifle to modify, I was first considering the recently-made fictional UAR airsoft AEG or perhaps an L85, but I've decided to use the Foreign Legion version of the FAMAS, which is already very close in appearance to the carbine Morita, so it should be a fairly easy transformation to make it look like a Morita without messing about with its functionality.
Morita Carbine:
http://www.imfdb.org/images/f/fb/HEROCarbine1.JPG
FAMAS Carbine:
http://en.wikipedia.org/wiki/FAMAS#mediaviewer/File:FAMAS_F1_with_bayonet.jpg