There are a few options for parking at Hobby Airport. I don't recommend parking in the on-site garage- it will be MUCH more expensive.
Reservations are not required, but there may not be any covered parking available unless you reserve ahead.
Covered or uncovered parking with a free shuttle. Generally less than $9 per day, and you may want to reserve parking ahead of time.
This is my personal fave- covered parking, usually around $8 per day, with a free shuttle.