Based upon the timing, it certainly sounds like it could be a false pregnancy. But only the vet can tell for sure.
A good mule can be worth quite a bit, but a mule foal tends to not be worth a whole bunch because they take quite a while to mature (think 5-6 years before riding).
Is your mare a polo pony? You could end up with a really good mule for someone willing to put the time into raising it up and training it. Most polo ponies that I've ever ridden were astonishingly nimble and smart (the very things that I like about a good mule).