Problem is most of them are there because blown head-gaskets or block damage. You spend $300.00 on a set of heads only to find they have valve damage or are warped.. Machine shops have long wait times and charge hundreds easy..

I learned a long time ago people only sale used cars when they think they are close to major costs, and they only wind up in junk yards when wrecked or need too much money for the repair.. It's almost always blown head gaskets or 2xx,000 vans with blown transmissions or crankshaft issues when not wrecked.. Even engine sellers can only get them at just under 200k and just mark them down.. You buy a 90k engine on the current market it'll barely pass compression tests.. Usually around 180PSI..

2008+ has really good parts, but if you can find one even with high mileage for $4,000 consider yourself lucky. I watch classifieds daily for my entire region and around 6k is below average even..

 

Head gasket jobs are easy, and rebuilding a head can be done very cheap and easily, if you have the knowledge.
There must be a lot of poor maintenance by you, I've pulled 200k mi 3.0s and 3.3ls around here that still run great with great compression. Maybe need headgaskets and valve seals, but its cheap to buy and takes around 6hrs once you know what you're doing.
My experience with the trans is if they have had the pack replaced they are usually ok, if not chances are a new pack will fix it. That's been 3 out of the last 4 I've picked up. 2-98s, 1-05, 1-08. All 4 were getting scrapped from trans being "junk", I changed the pack on them and 3 were back on the road. 1-98 had 368k, the trans was actually junk in that one. I picked all 4 vans up for @2400. Paid $1200 for the 08, $500 for the 05, $500 for 1 98, and $200 for the other.

 

I get at least 280k on all mine before I just get rid of them.. Heads aren't easy when they warp and there is barely a market for engines let alone heads, and machine shops charge the price of another engine or even van to surface the head; valve seats you might as well scrap it.. The blocks don't have replaceable sleeves either so your number is eventually up either, and they also warp..

EDIT: I found a lot of 3.3 4th gen engines for $300-$400 130k'ish on CL.. 3.8 used market is almost non-existent.. I'm going to start keeping two PCMs for each on all mine..

 

Wow, I have never got a motor with warped heads, nor have I ever warped any. To me it wouldn't matter as I would just machine them myself if they were.
The motor I have tore down in the garage has 328k on it and still has the factory honing on the cylinder walls. That one is getting some Frankenstein work for a friends 2nd gen, should be fun.