I wouldn't go custom at all, I already posted my reasons for that earlier in this thread.
AI AT (in .308): Wolverine as of today, $5,199
Custom (in 300wm):
Action: $600 (rem 700, $700 minus say $100 for stock and barrel after takeoff)
Timney trigger: $200
Match Barrel: $450
AX AICS Stock: $1,495
Pic rail: $180
Corlanes Gun smith work: $675 (action truing $275, bbl install $250, tactical bolt handle $125, or if you know a good local like I do, then it's a better price)
Total: $3,600
So, for a difference of $1,600 you can buy an AI. Or build one that will shoot just as well (maybe better depending on the barrel). Unless you take a $1,600 hit on the resale, there really is no reason to go for the AI over a custom rig; unless you are deployed in extreme weather conditions. In either case, if you shoot it, you will burn through the barrel. You will always take a hit of at least $450 by shooting it. If you don't shoot that much I do not know why you would be looking into any of these guns!
Once you shoot through the AI barrel, replacing it is like a minefield. If you opt for an AI replacement barrel, you're looking at a US price of $675 through eurooptic (not including paperwork and etc to get it up here). And that's to keep it 'spec.' If you opted for a true flight barrel for eg, you've made a franken gun, and resale there will hurt you; negating any of the 'spec-ness' of the AI rifle in the first place.
Of course, if a guy went with a savage 110 hs precision in 338 for $1,500, then he'd have a lot of money left over for glass and ammo... And you could do your own barrel conversion kits (as long as the bolt faces matched) thanks to the barrel nut plus a go and no-go gauge. I think you can even swap the bolt face due to the savage design. Not as easy as the DT.
At any rate, it looks like the price to pay for a DT bullpup and 2 caliber conversions is just shy of $10,000 for a DT SRS (corlanes), and that is a lot of cheddar. I am a huge fan of portable sized rifles that fill big shoes, but not sure if I am a fan to the tune of $10g's.