I've used dyno's for a decade plus now and they're certainly worth it if you're tuning a vehicle (and have a shop who knows how to adjust a tune, or can remote in with the tuner). I've used both RWD and AWD for different cars, most recently for my fully built 335xi. Most shops charge the hourly shop rate for dyno work and you get however many pulls you need until you get it sorted or want to throw things. Throwing usually comes first

For the X you'll need an all-wheel dyno. A quick google search should yield some results in your area. Call them each and ask what they charge. Are you tuning it?