A wave sailing trip mean that your principal occupation is going to the spot.
What are some good place to stay regarding the location, the fact that you have a windsurf gear and the budget?
I'm guessing from this question that you plan to sail in the Ho'okipa area.
This is not really the place for specific recommendations (they get stale fast), but I can make a few generalizations based on my experience that might help.
There is plenty of accommodations in Paia town and Spreckelsville which would be closest location to the sailing beaches. However, I haven't seen a room for $100 per night where I would be comfortable sleeping -- that's a very high-demand area, so the market seems to be able to support a fairly high rate for fairly poor accommodations.
Of course, your comfort level, how far in advance you book, and luck all play a role in whether you'll find a room you're happy with in Paia.
Vacation Rentals without Permits
The county of Maui has been transitioning to regulating all vacation rentals. I don't know exactly what the implications of renting a place without a permit are. However, before you rent a place without a permit, you should probably figure out what problems that might cause.
In my experience with about 10 different rentals, there was a correlation between poorly maintained rentals and rentals without permits. But I'm sure there are many properties that don't follow that correlation.
Haiku, Makawao, and Kula
I have found that accommodations in Makawao, Haiku, and surrounding areas are much better value than those in Paia. Also, the restaurants are better (in both value and quality) than Paia. The downside for Makawao in your case is that it will add about 20 minutes commute to the beach, but it still might be a better way to go.
Booking in Advance
Many people book the same accommodations year after year in Maui. What this means is that the very best accommodations in every price bracket tend to be booked a year or more in advance. The less in advance you book, the fewer good options remain.