What I do is search for related items online and price around that. I went on etsy to sell a large blue marlin painting I did and I found similar paintings that were the same size. I kind of priced around them.
It's a very good idea to see how others price their art online. I ended up pricing my painting at 400 dollars which is a lot but a good deal comparatively to other paintings.
I'd charge between $400 and $600 for a traditional work that's about 2' per side. But I'd probably get better at art before I tried selling to people. That's a starter price for a really big work, but people who pay that price for art are rather unlikely to pay for fantasy art that is so simplistic.
Oh! So they're large pencil drawings, not necessarily "posters". I gotchya. I was thinking you were printing posters.
As with any commission or sold artwork, definitely factor in the time taken as well as how much you think your skill is worth. You can always adjust your prices if it doesn't work out; just don't undersell yourself.