Personally, I wanted to spend little on acoustics and furniture, in order to spend more on gear, which I find more exciting. So here's what I did:
1. Furniture. I bought everything in a department store, like home depot.
I placed hollow door panels on heavy-duty testles (check their max. supporting weight!). The trestles are height adjustable, however you can't adjust them once everything weighs on them.
Door panels are cheap, lightweight yet rigid. They make a desk that is about 2m wide, and 65 to 93cm deep, depending on the door model. With enough weight on them (such as your gear) they won't vibrate. You can also staple or glue sheets of cork on their top surface to avoid reflections. The look and feel is rather classy. I used the thickest sheets I could find, which is 6mm.
Instead of trestles, you could use a pair of sturdy keyboard stands. Check how height affects span, though. And check the max weight the stands can support! It can vary greatly from one model to another.
I set the height in order to work standing, and got a high chair to rest my buttocks against in still a half-standing position (something like this:
https://www.musicstore.de/en_BE/EUR/Mey-Chair-Systems-Stehhilfe-AF6-Kl-Black/art-REC0008420-000 or like this:
https://www.musicstore.de/en_BE/EUR...lfe-AF-SR-KL-AH-Black-blue/art-REC0008407-000 , except more expensive - the chair being one of the few things I spent more money on, because I want to preserve my back).
If you can't fit a 2m-wide desk in the room, then you can go for a thick wooden panel instead of a door. It may not be as rigid, but since it would be shorter, it might not be a problem. Ikea sells desk tops and trestles too, and some of their tops are cheap. Ikea sells regular legs too, which you might prefer to tresltes or stands, but it's a less sturdy solution.
2. Acoustic treatment: I got a bunch of used mattresses (plain ones, not spring ones!), and strapped a double layer of them across the walls to tame most of the bass. It's not perfect, but it's pretty decent and cost nothing. Heavy curtains and sheets of cork do the rest where there are no matresses.
To strap the matresses to the walls, I asked a friend to sew velcro tape on light-duty straps, and placed wide heavy-duty adhesive velcro on the walls (which only comes in ugly black, but it's hidden behind the matresses anyway). And now I can hang my spare cables to the straps... It's pretty handy.
Of course, I abundantly sprayed those old matresses against odors and vermin - a few cans worth of spray was no luxury.
Now if you want diffusors, you can just use bits of wooden boards or panels (leftovers from other works, if you have some lying around), that you'd cut into erratic shapes, and then screw or nail a few layers of them, erratically arranged, on top of each other, on a back panel for support, or directly on the wall if you don't love your wall too much. The result is pretty heavy, but it's cheap or possibly free, too. You can also combine a diffusor and high frequency absorber by placing a layer of absorbing material (like one of those cork sheets) behind this wooden bits salad, over the back board.
The only thing I might have overlooked is the floor/ceiling mode. I've heard you can hang a light rigid panel parallel to the ceiling above your listening position, and the low freqs will consume their energy making it vibrate (it must of course vibrate silently, hence the hanging)... But I haven't tried that trick yet.
And the only thing a bit costly (besides the chair) was some thick carpet, because the floor was bare concrete. But if your room is smaller than mine, and its floor is already decent, I'm sure you could find a bunch of rugs for cheap.
(In my previous studio, the floor was ceramic. So I got some end-of-roll leftovers from the carpet store and roughly cut them to the shape of the room. It was a quick and dirty solution that worked just as well. The thicker the better of course.)
See for yourself:
I confess those white cylinders are "real" bass absorbers, but I use them more as speaker support than as acoustic treatment
. They're also something I had to opportunity to salvage - I probably wouldn't have purchased them new, even though they're affordable.
Last bit of advice: Avoid acoustic foam if you intend to use that room for years, as it doesn't age well and tends to degrade into a sticky dust over time.
[EDIT: Oops! This doesn't really answer your question, does it? I thought you were asking how to make a studio on a budget... I should read more carefully. :-/ ]