Unfortunately the only way to tackle long grass is to add weight, which requires 3:1 gearboxes to take the strain off the electronics. I do drive my HL equiped tanks in long grass with standard ratio gearboxes - but I have to do so with care, and I have to accept that it will not perform as well as a more expensive tank in those conditions.
I'm going to quote tankzone prices here just because I'm lazy (and they include shipping):
Reinforcement bar kit 12.95
Metal suspension kit 16.50
Metal sprockets 14.95
Early / Mid idlers 12.95 (or Late Idlers 14.95)
3:1 gearboxes 39.95
= 97.30
Hi grade metal tracks are 79.95, but I wouldn't buy them witout first upgrading the gearboxes.
As I posted on RCU, If you have the older style press in axles then you will need the metal suspension set, as the plastic arms usually break at the factory when the chimp assembling them pounds them in with a sledge hammer O.O if the wheels are held on with screws you won't be able to use these with the metal arms (unless you get new screws and thread cutting tools
)
The asiatam hulls are nice, but I'm not sure how much of a mechanical improvement they offer vs adding the reinforcement bars (plus Brels suggestion for bracing the rear of the chassis) if stiffness and better suspenson mountings are all you need. Having tried both routes on 2 different tigers, I personally think the asiatam chassis is only really worth doing if you want to add better electronics that take up more space, or lots of cosmetic metal parts and extra batteries that add extra load on the suspension springs.
(or in my case you have the matorro Tiger / SturmTiger with its poor gearbox that can't be easilly swaped for something better
)
have estimated it to cost around £100 which is handy as thats what i should have to build it.
pretty much out of my budget! and your exactly right asiatam hull is WAY too much 
Oh and it took exactally 24hrs to arrive from ordering it O.O
)