I have a Sigma Sport PC7 that I got from either Nashbar or Performance Bike for $35. That model is discontinued, but there seem to always be cheap HRM available around that price, and you get both the watch and the strap. I use its strap with my PT head with no problems, and I have read that the reverse (PT strap -> watch) should be fine also.
The PT head can be put into HRM mode (kept awake by HR input, not speed or power) and used for logging your HR versus time. 2 receivers (PT head in a pocket and watch on wrist) shouldn't interfere with each other. Another alternative is to strap the PT head to your wrist or put it on a chain.