"How difficult would it be to make this bi-directional, and how much bandwidth is left over? "
Making communication bidirectional is easy, but probably not very efficient because of the acknowledge.
Regarding bandwith, for sure there is still some space for other datas. However I am sure about the current data loss I have. I am under the impression that the more the bandwith is high, the more there is character loss. This is one of the reason why I added to my protocol a crc to check data integrity and to avoid this problem.
"if this works out cheaper than a normal 2.4 Ghz radio set ..."
A xbee pro is approx 40$ and you'll need at least 2. Then add the arduino + wee 34$+20$ + hardware and you'll be at 140$ or more. It seems that you can buy 2.4Ghz small plane for that price. So I doubt that you can't find cheaper.
"I noticed on your website that you had your first flight! Congratulations, it seems to work nice! "
Yes I did yesterday. The plane flew with no suprise even while I was flying far away. This is encouraging to continue developments.