Okay, this'll be a bit rough.
Maelstrom games charges £13.12 for the
IG codex. £56.88 remaining.
Common
IG units, their points costs, their price at Maelstrom, and points/price ratio
Chimera, 55+, £17.50, 3.14. I like to upgrade with stubbers for +10 points, p/p ratio 3.71
Battle tank, 150+, £21.87, 6.86 Good value
Valkyrie, 100 or 130, £30.62 , 3.27 or 4.25
Sentinel, 35-75, £13.12, 2.67-5.71
Demolisher, 165+, £26.25, 6.28
Basically to make the cheapest possible Guard army you want Veterans as troops with as many battle tanks as possible and maybe some valkyries.
You probably want a command squad but the actual kit costs. Use bits and stuff to build an officer out of an ordinary guardsman. Use the guardsmen that came with your jungle fighters kit to build the command squad.
Company command squad 50, 3 special weapons (your choice of flamer or grenade launcher) total 65
Troops: Two vet squads, again using models from your jungle fighters kits. Ordinary vets plus three of either flamers or grenade launchers cost 85 points. I don't think much of doctrines but it might be necessary to add either forward sentries or demolitions (+30) to make up points.
points cost on troops 170, total points spent so far 235. 765 points to go.
That's 25 men in total, so you'll need to buy another squad box for £13.12. £43.76 remains. This happens to be almost exactly what two battle tanks cost. Bare bones that costs 300 points, making your entire army worth about 535 points and now we have two cents left and we haven't even paid for shipping yet. You can throw tons of upgrades on everything to chew up points, but I think that no matter what you do with your budget you'll end up a few hundred points short of your target with this budget. You could potentially build a 500 point army on this amount of money, but 1000 is out of the question.
For 500 points you could possibly build:
Company command squad, 4 grenade launchers (70)
2x vet squad, 3 whatever (85x2=170) and 1 chimera (55)
Leman Russ Executioner with hull lascannon (205)
500 points, needs to buy 1 squad box (£13.12), 1 new type Demolisher kit (£26.25), and 1 Chimera (£17.50) for £56.87. Add in the codex and you get £69.99 plus shipping.
I don't know what heavy weapons team he's built if he's built it at all so I didn't include it in the list. It wouldn't be too hard to shuffle points around to fit it in if you want. The easiest thing to change is turning the Executioner into a cheaper battle tank to put more points into upgrading the squads. I don't know what weapons he's built his regular jungle fighters with but the additional squad box plus his leftover bits should be able to cover whatever he hasn't built.