Hi Poshm: There's no lower word-count limit on ebooks (at least not self-published ebooks and those running through many small ebook-only 'publishers'), though as has been said, you should play fair and price accordingly.
Novellas by 'unknown' authors are unattractive to trad publishers for various reasons, so if you find there's a treebook demand, you're likely to end up self-publishing using a PoD press. In this case, you'll find that the minimum of 100 pages applies; a lower limit set by the printers themselves for various technical and costing reasons. (Page signature [the actual size and shape] of pages also comes into play, but there's no need to consider that yet.)
I've known folks get around this by legitimately padding their novella work with their own short stories or another novella and perhaps a few shorts by other authors. If your sales are likely to be mostly author-circle, this will increase circulation, of course, but the profit will be split among co-authors.
The advice given here is good -- NEVER pad a story itself. A piece of fiction should never be made to carry more weight than it can comfortably bear. Word economy is always the way to go. If you've self-edited, I can almost guarantee that your piece might still benefit from some judicious pruning.
My own house published a novella a few months ago because I thought it was of particularly high quality, and I made up page count to 160 with the addition of three short stories (about 5K each) by the same author and some superb inside artwork, which produces a respectable package of satisfactory heft.
If you'd like to take a look to see how it worked out, just drop me an email and I'll send you the ebook version of
Allakazam! by Daniel Abelman. You can see details on our website at
www.bewrite.net to see if you're interested.
I'm afraid I only have a PDF version until our titles are converted to other formats next year. Because we had to play with different fonts in this piece, it doesn't lend itself to easy conversion to other formats for ereader use, but it'll give you an idea of what can be achieved, even if you just flip through the pages on your PD or laptop.
Cheers and good luck. Neil
ntmarrATbewrite.net (use the @ sign)