Compared to producing a full length novel, shorts take less time per word. Typical shorts have one twist and only a handful of characters at most. This could be anything from a thousand words up to ten thousand (more for a novella).
With a less complicated story, and less 'writers fatigue', editing is much easier. Few authors bother with substantive edits. Some will even skip line edits, and go for a quick copyedit. This keeps costs low. Using a critique partner (or circle) can reduce these costs down to just time.
Covers can be done in house, and if you've got the talent to do your own (with own or stock photos) then covers can be cheap. I prefer professional covers. It isn't simply taking a picture and putting text on it. A good cover designer uses paid rather than free fonts, gets bulk rates on stock images and understands composition (including typography, lighting, colours, focal points etc).
For a full length novel, I always advocate using an artist who has a style you like, and experience in designing books (even if just a practice portfolio). I used Nadica Boskovska for Dead on Demand, and Laura Wright for Can't Sell, Won't Sell. The former did a 'from scratch' illustration for me including several rounds of pencil designs then colours. I own the copyright to my cover - I can use it when and where I like with no further payment. Can't Sell, Won't Sell was done using stock photography with all the licensing restrictions that implies. Both were 'to specification' i.e. I told them what I wanted.
Pre-made covers can be both stock or illustrated, but the former is much more common (99.5% of the pre-made market). These can cost anything from $25 up to around $100 depending on stock (and if a wrap around is supplied). Compared to the costs of 'to spec' design (which is highly variable, but can be hundreds or even thousands) it's very cheap. For about £16 at today's exchange rates, I could have a professional looking cover supplied, possibly within minutes. Bulk sales make this even cheaper - Go On Write do some great covers in a 20 for $450 bundle. Big investment, but at $22.50 each it isn't quite so hard to recoup costs.
I think pre-mades are ideal for short stories. Most shorts will be 99c (or possibly multiple for 99c in a collection). Unless you write erotica or are famous then you won't make much above this price. If you're very good, then you can apply to have it included as an Amazon Single (which is an editors pick of stories 'told at their natural length' under 30k words). This gives you a 70% royalty even at the lesser rate.
Whether or not you publish as an Amazon Single, you won't be making much per copy. That means alot of sales to make it all work. I'd guess most of us could write and edit a short in 15 hours, or approximately 2 days. At the minimum wage rate that's £94.65 invested in time. Add in your pre-made and you need to earn £110 or so to justify publishing it.
At 26p/ copy at the UK's 75p minimum (or 35c at the US 99c minimum which is more like 24p) we need to shift 423 copies for it to pay off. That's not easy, nor is it insurmountable taking a lifetime view. The big win is the extra digital shelf space, and keeping your name in front of readers who might go on to buy more lucrative ebooks at the $2.99+ 70% royalty rate. Even if it's an Amazon Single you'll need over two hundred copies to sell to make it pay off.
I think pre-mades are a good bet here, and my personal preference is to write a book to fit a cover rather than have a loosely fitting cover chosen afterwards. The more vague covers work best as you can shape the story as you wish, and just match the protaganist to the person depicted on the cover.
Have you used pre-mades? Was that for a short or a full length? Are you happy with sales?