It's worth $500 to you; to someone else, it's hard to say.
Since this was a fixed-price sale, apparently negotiated with the seller, there's no easy way to determine a definitive market value. If there had been a previous sales history, or if it had sold via an eBay auction, that would have provided more context.
The module was written by Richard L. Seaborne, who apparently went on to produce the Tower of Myraglen computer game, which was released in 1987. This 1984
RPG module appears to be a self-published or micropress production, likely made in very limited quantities. The typewritten layout, stapled binding, ASCII-style artwork, and crude hand-drawn maps are typical of early amateur, tournament, or fan-published
RPG materials.
Did the seller provide any provenance or additional details on it? That would help in authenticating it.
Items like this are especially valued in the vintage
RPG collecting communities, like those on The Acaeum, particularly when tied to known creators or early cross-media projects. While the computer game itself may not sell for a lot (about $43 on average), this module is probably worth more, due to its rarity and the fact that it appears to be a precursor, or even the original source material for the PC game.
Without broader sales data, it’s difficult to assign an estimated value, but given its obscurity, provenance, and potential appeal to collectors, your purchase price doesn’t seem out of line. That said, if you were to offer it for sale at auction, without any reserve or high starting price, the final sales price could vary wildly depending on how many collectors were interested in it. I would suspect that it wouldn't normally sell for that high of a price, though.
It would be interesting to see if any of our Acaeum members have a copy of this and/or what their opinions are of the estimated value.
