I think the idea could work, but the initial suggestion is too imba.
Xar runes are too cheap (I don't have one yet, or many high runes > Zod, but I still wouldn't want to get them that easily.) El runes are way too expensive; you can find them in the Den of Evil at
clvl 1, but you'll never be able to afford to buy them at any point in act 1 (even though you'll find plenty of El, Eld, Nef, Tir,, etc. so it would just be weird, not to mention that merchants will give you a lot of money for those low runes.)
Better solution is some sort of exponential curve.
El rune -> 50 gold.
Xar rune -> 10,000,000,000 gold.
(I've been playing this mod for several months and really don't spend gold that much, (some gambling cheese but not often since it's not all that enjoyable) and have 32 billion gold in shared stash, so that means I would be able to buy 3 Xar runes after spending 4-5 months of play time, which doesn't seem too imba, and it would still be faster to make Fauztinville runner to get runes, so there's less loss of incentive to try Fauz)
Problem there is that max gold in stash for level 120 character is 3,050,000, and I think you can carry about 1,000,000 more so max cost of Xar that is actually buyable would be 4,000,000 which is too cheap.
I'd like to see something like the old D1 economy, where you have the option of either:
1. Be able to buy decent items from vendors as you go along, but they are expensive and you barely stay afloat financially, but they sell better stuff than what drops normally.
or
2. ignore the vendors, save, save, save, use item drops for your equipment and just go to vendors to repair them, but then you have a ton of gold at the endgame to purchase something like a book of skill or high rune or really, really good items.
However, even that won't work well with PlugY (sigh.)
If gold sharing can be disabled from the mod (like you did with skill and stat re-allocation) then I think it would be better (though still not great because of that 4 million max gold cap.) so the economy also needs to be scaled down quite a bit (i.e. no gold sharing and it takes quite a bit of playtime to accumulate 4,000,000 gold, and scale goes from El @ 50 to Xar @ 4 million) Oh, and you lose gold you carry if you die, so better not die after you accumulate 3 million
) Main problem now would be same reason why I never had level 99 character in D2 or 7 Lances or whatnot - it would just become endless cow runs to get gold so that you can buy the rune you want, but by then you're too tired of your char to make the cool runeword you were drooling over, so the rune becomes another twink item.