You mentioned a few requirements:
- suitable for soldering
- suitable for writing
- less than $50
You have several options for a soldering table. The anti-static laminate ones I used in college looked just as good when I worked in the computer repair shop on campus as they must have when they were brand new. As long as you don't touch the tip of your soldering iron directly to the surface, you're not going to burn the table. In my experience, dripping solder cools and solidifies immediately upon contact with laminate, and brushes off easily.
I've also seen soldering tables with MDF tops that took a lot of abuse from hundreds of students over the decades, but they were still in reasonably good shape despite some "character." I'm pretty sure the tops were intended to be replaceable.
If you plan to do a lot of soldering, you may want to consider a surface that can be refinished, or design your table with a sturdy flat subsurface and an easily-replaceable top surface.
Of course, if soldering isn't going to be your main activity on this table, you can also lay a board or mat on top of the table anytime you pull out your soldering iron.
Pretty much any surface can be made into a writing surface if you're writing in a notepad, but if you're talking about laying a piece of paper directly on the surface, that can sometimes be an iffy proposition. Personally, I wouldn't give this requirement much weight unless you're talking about building a drafting table. Pencils work great on smooth surfaces, but a surface with a little texture works better for ballpoint pens than a perfectly smooth surface. If you've ever tried signing your name on a piece of paper on a very smooth surface, you know it can be an exercise in futility. Of course, if your material or surface is too soft, you can end up etching the surface when you write on top of it.
Your budget gives you a lot of options on both the new and secondhand markets: MDF, hardwood-veneer plywood, laminate, or even solid, reasonably hard hardwood if you can find a local lumber mill. In terms of selecting an appropriate species of solid wood, see What minimum level of hardness (on the Janka scale) is appropriate for a desk top?. You may even be able to find some appropriate material at garage sales, on craigslist, etc. If you have a university nearby, you may be able to find a lab table from a chemistry lab. If you have a Habitat for Humanity ReStore nearby, you may be able to find granite or laminate countertop material pretty cheap.
A previous version of your question also mentioned sheet metal, but regular steel rusts if not sealed, and stainless steel and aluminum are very expensive. Also, thin metal will dent easily. Thicker, more dent-resistant metal is even more expensive.
You didn't mention what plans you have, if any, for finishing, but a durable varnish such as polyurethane will also help protect your table from abuse from soldering and writing, as well as any spills. It will also help protect against minor dents.