This is also one of the experiences I dream about which I hope to fulfill soon. I remember researching about Hot Air Balloon ride in Bagan I checked and price is $310 to 380$ per person. Actually watching the balloons over the temples is also romantic for your GF you should go onApril :) I haven't done it but my friend did the Balloon ride she said it is the best travel experience so far :)
Thank you for your advice @joyceann2 ; I hope you also have the chance to experience it anytime soon