Mainberry - chill the beans.
No-one in here has said anything about your tape - other than newcastle 86! - and you would have a very, very difficult time in a court of law to prove that he was NOT joking anyway.
But, I digress, your tape is not a family heirloom, and you would be lucky to get more than £10.00 (that's ten UK pounds) for it - here's why.
1: If it's not good enough for Greg - it's of no value to a collector (ie both would be interested in musical rarity value - and as the music is not that disimilar - then it is not valuable).
2: It is a promo sound engineers monitor mix - this means it is no more than a promo tape of the final mix - not an alternative mix (or better still an alternative take).
3: It is a casstte tape, and tapes do not retain their value in the way a vinyl or digital disc does. This is because tape is destructable (it is easily destroyed/ stretched/ chewed etc)and degrades over time. Even if it was unplayed - after 10 years, the quality begins to disintigrate remarkably. 10 years after that, it will have done so again. Considering your tape must be c25 years old, you would really notice the difference. By contrast, a 25 year old acetate, would be in surprisingly good nick.
4: It is not a sexy item. It is not a blue vinyl - or 12" picture disc. It does not come with a unique cover or exclusive cue-sheets. By your own admission, it looks like a bog standard tape, and seriously, who would be interested in that?
5: Finally, Crazy Little Thing Called Love, is in the top 10 of the most popular Queen tracks ever. Any Queen fan worth their salt will already own this in at least a half dozen different variations or formats.
Even if you do not own the single, or the Game LP/CD, CLTCL appears on almost every Queen compilation (if not on general assorted NON-Queen compilations) - so even the most casual of Queen fans are familuar with this work. Add to that, a myriad of free internet downloads - and you can see why the spectrum of Queen fans pretty quickly lose interest.
Believe me, if it was of any value whatsoever, then GB would have bitten your hand off for it - but he has passed for good reason. Therefore, other than someone who may desire the tape as some sort of curio, it really isn't that valuable collecting wise.
I hope this has been informative and useful, and may help you in your search for a new owner.
"Listen to them. Children of the night. What music they make."