Just what is special about the resist pens used to draw directly on PCBs before etching? Would a permanent marker work as well, or some other kind pen or marker? Naturally I have in mind something cheaper but good enough. This is for one-off handmade projects, single or double sided boards only.
A permanent marker will work but the finishing may not be that very great and you might want to give it a solder coat later. A resist pen just forms a thicker layer than a marker would do, giving it more isolation from etching chemical and a better finish.
I have used it couple of time If I am lazy for other long method where I get the circuit done on a PCB designer and get it printed on photo-paper etc. But for a good finish and a complex circuit I would suggest this method of etching is the best and reliable. I have been using it since years and never had a problem.
It's mostly about how much ink they put down - certainly worth testing some standard permanent markers on some scrap material. Many shops that sell markers have testers you can try so maybe take a bit of copperclad with you & try a few.
I used to get very good results with ordinary cellulose paint, applied with a fine paint brush. Thinning the paint a little with cellulose thinners helped if I needed narrow tracks. It worked better than the resist pens and was much cheaper.
"pcbprt - Experiments in inkjet PCB printing" by pascal. Some inkjet printers can print directly CD and DVD. They can print on copper-clad FR4 to make reasonably good etch resist. The main trick seems to be baking the freshly-printed boards to dry out the ink and get the dyes/pigments to stick to the copper -- otherwise the water-based ink immediately washes off as soon as you drop the board in the etch tank.
I've had success with some overhead-projector-transparency pens (remember OHPs? :). The dark blue and brown Staedtlers worked best IIRC, the purple one not very well at all - remember to use the "permanent" ones, not the water-based ones.
A bit of trial and error is the order of the day!