Sorry, rather late to this but I was also looking for an MTB power meter the other day and found this:
https://www.iqsquare.com/Now from my understanding there have been issues with this Kickstarter project in that their first attempted to produce a "cheap" power meter did not work, then the second bit of thinking towards pedal based items. Currently, they are taking orders for MTB SPD like items at E229 for a single sided item with shipping this month.
While that's a good price.... well it was until the recent fall in the £, frankly I won't be investing until they have actually shipped a number to prove they got it right this time. However, worth keeping an eye on I think.