Hello,
I am a senior telcomm/datacomm coder and I know USSD and SMS-C interworking protocols, I can code over MTP or Sigtran depending on your needs. I can also provide you references and assist on purchasing USSD-Gateways solutions, but the budget you've set only pays the prototype or either a conection with a world wide USSD gateway provider, and depeding on the network we need to test the messages.
Your project will require about 800 bucks to see daylight if you're intereting on having something solid and usable, and if you choose me, the 250 I am asking now will be taken as part of a payment for the 800 bucks total.
please keep me posted, I've been doing his ussd, bulk/sms thing since a while
regards
alex