As an upholsterer working in the Riviera Nayarit area, I can tell you that as with most things, you get what you pay for. There are varying price factors having to do with upholstery. Most stuff looks good when it is new, the question is how it holds up through time and, in the case of slipcovers, repeated washings.
I have had "Sunbrella" fabric brought to me by clients that was purchased in the States, Canada, or Mexico, that in fact was not Sunbrella, but sold to them as such, and which the color faded from in 6 months. There are many different qualities (not thicknesses) of foam, with corresponding price difference. I usually put an inner cover over the foam and dacron batting of a couch cushion, for instance, so when the outside cover is removed for washing, the fuzzy batting doesn't get stuck in the zipper (which usually breaks the zipper) and the foam and batting are all held firmly together. This adds somewhat to the cost, but most of my clients opt for it. Also, are the inside seams finished off, or will they continue to fray away every time you wash them until the seams rip out? I can assure you that if the quoted price for a job is much lower than anyone else's, (and if they don't give you options for foam quality, you can bet they are using the stuff that goes flat in a short time), you are likely getting inferior materials, or less than high quality work.
As for tradesmen not showing up when promised, or taking months to do a job they told you would take a week, well, some people just don't know how to do good business, and reap the consequences. Although you won't use them again, some other unknowing sucker will come along.