I was googling around, and the cheapest I could find a copy of the expansion(used) was for $25.95 (including shipping). That is a bit more than I would hope to pay for an expansion to a quite old game. I could find it at a certain bay that has pirates, but that is not really my thing.
In the opinion of the people around these parts, is the expansion worth the cost to play the few added features in the expansion?
Hellfire no longer sells officially so it wouldn't affect anyone if you visited the bay with pirates.
Anyway, I think Hellfire is a great expansion for single player. It adds a lot more re-playability than the original Diablo by adding Nightmare & Hell modes for single player, and the catacombs/caves/hell are remain unlocked if you discovered them in previous game(s). The new areas and monsters are nice, as well as the new spells. The new sounds are of low quality, and there are a lot of bugs. Hellfire was made with 1.04 Diablo, so it lacks the bug fixes in 1.05 - 1.09. I don't know what most of those bugs are but I haven't really noticed any.
I think Hellfire is worth it if you plan on playing single player long-term. I've been playing it for awhile now and I am quite enjoying it.
There is also The Hell mod, which is an extension to Hellfire (http://www.diablothehell.narod.ru/main.html). The mod is highly praised by a lot of players and is often considered the best mod. I haven't played this yet, but it fixes many bugs and tremendously enhances the gameplay experience.
I am no sure how long term I will play Diablo 1. I play a lot of D2 SP already. I fully intend to play through it at least 1 time with each character. I was playing on Bnet last night. Seems that almost everyone I met though was a duper or cheater. I even had warning messages from the game something along the lines of "xxx had a detected duplicate item". The few I saw that weren't cheating didn't speak English or tried to PK me.
I might look into Hellfire and hell mod, but it is unfortunate that it is SP only. Even though I play mostly solo, I still like to play the occasional group game.