My thoughts. Take or disregard as you will...
Once I figured out it was auto-selling equipment, I didn't have too much of a problem with it. But I agree that there should be multipliers for buying/selling and that won't go well with auto-sell everything.
Auto-keep would work, but I found that rather annoying in X-Com... especially when getting close to a base's storage limit. A more advanced option would be a post-mission yard sale: a list like the current Purchase screen showing amount of each item gained and the ability to sell it then and there, before going back to the geoscape.
(Listing how many of each item is already at the base and the ability to transfer to other bases would be useful additions.)
To me, the X-Com manufacturing system always felt like a cheat. I think it was the laser rifle (or something like that) that didn't use any elerium on construction, was relatively quick to build, and sold for a good amount over the cost. After about four or five months, the disposition of the nations was financially irrelevant.
So I like the concept of building costing more than buying. (I mean it costs less to buy a TV than it would to make the same thing from scratch.) But I do agree the multiplier is a little extreme.
As a player, I would lobby for a prototyping system: No researched items can be bought until you've built one. (You could make arguments for and against the sold items being able to be purchased back before prototyping.) The prototype costs some factor more than the base manufacturing cost (like x1.5 or x1.2).
Once you've built the prototype, you can build more yourself for the base manufacturing cost and/or the quantity of the item will increase in the Purchase screen.
(For simplicity, I'd argue that prototyping a weapon automatically prototypes the basic ammunition. Alternate ammunition like plasma grenades/rockets may need to be prototyped.)
As for the manufacturing costs, I agree 190,000c for a 19,000c item is extreme (especially early in the game). Maybe a sliding multiplier for manufacturing and prototyping costs using the base cost...
A very rough table:
Base Cost Manufacture Cost Prototype Cost
0-999c x10 x20
1,000-9,999c x5 x8
10,000c-100kc x2 (minimum 50,000c) x3
+100,000c x1.5 (min 200,000c) x1.8
(Now +100,000c items would cost less to manufacture than to purchase--given a x2 sell multiplier, but I'm assuming items in this price range are ships or high-end items that would require additional components and long production times that would make production for profit inviable.)
For example, I think a Bolter Rifle has a purchase price of 4,500c. Assume that the purchase price is x3 the base price, so the base price is 4,500/3 or 1,500c.
The prototype of the Bolter Rifle would cost 8 * 1,500, or 12,000c. After that the manufacture price would be 5 * 1,500, or 7,500c. (But after being prototyped, more would appear in the purchase screen over time and only cost 4,500c.)
The manufacture and prototype multipliers could also be tweaked based on the difficulty level.
Thinking about it, you could do away with the whole workshop/workers system. Instead, manufacturing capability is completely “out-sourced†and tied to nation happiness--the better you do, the more external resources you have. But that could get complicated from both coding and user angles and is a whole other topic.