I have a boxset of the first and second phases of the Hitchhiker's
Guide to the Galaxy on audio cassette that was released in 1988. The
4th tape has a misprint on it; the number 1 is printed on the b side
of the tape while the a side of the tape has 2 printed on it.
How much money do you think this might be worth on ebay for example?

I'm not sure how much money you could get on eBay as people are
bidding somewhere between $2 to $76 for cassettes of the show. The $76
is going to a three tape set. The tapes are hard to find as they are
out of print and only available from a small group of sellers and
library systems. Do you have used or new tapes? What condition is
everything in?

If I was to advertize this on eBay I would stress how hard to find the
box set is and mention the error on the fourth tape to emphasize how
rare it is. I don't think fans of this genre care too much about
mistakes like this though. Judging by the content of the series, some
might think the misprinting was intentional.

I did see a price of $325 from a seller on Amazon for the 1994 boxset
of Hitchhiker's tapes!
http://www.amazon.com/exec/obidos/tg/detail/-/155800758X/ref=ase_hologratalesthemA/102-6685720-0660916?v=glance&s=books
A 1998 version of what you have is going for $55:
://www.google.com/froogle_url?q=http://www.opengroup.com/lxbooks/056/0563557664.shtml&fr=AL_rRyKXOGH1B8af5NLWCORpw9X1UNSYKktMPoxmCjyBAAAAAAAAAAA

I would say on eBay you could get from $25 to $100 for this. On
Amazon, you could set a higher price but you could wait a long time
and it could sit there. A Sci-fi convention probably would be the best
place to sell this. Either way, I wouldn't wait as the movie is fresh
in peoples' minds and some might be willing to put up the big bucks.

The BBC online shop have just reissues The Hitchhiker's Guide to the
Galaxy on Cassette/CD and the TV series on DVD. I'm not sure if they
are availbe in teh States yet.