Sounds too high to me.
The frame only has potential value. Without someone prepared to move it, renovate and reglaze it is little more than rubbish cluttering up an untended site.
Now if you offered to remove it, and weed, clear and dig over that part of the plot, the site management might see that as a win win. The plot will be partially cleared and easier to relet, and not cluttered with rubbish. Give it a go. You might get it for nothing, albeit a couple of days work clearing and digging.
Not all plot holders want a greenhouse. Without one, I can go to my plot often or less often as I choose. With a greenhouse you have a far greater commitment to get along often, for the purpose of watering, ventilation to prevent over heating, and sometimes closing up to protect delicate plants. I have taken on untended plots in the past and would have been less happy if I'd also faced the extra work of dismantling and removing an old greenhouse frame.
Start low and if necessary offer a bit more, but my opening would be an offer to take it away in return for tidying that part of the plot where it currently stands.