My Dear Lady X,
If your bf won't go for the catbird heart ring (silly bf), maybe he'll go for this Scosha heart ring with a teensy 1mm diamond. At $129 it's even almost in your budget.
In other news, Scosha now does custom wedding + engagement rings. WHY WAS I NOT INFORMED? I was so busy lusting after the bracelets (which, I finally bought one at the Scosha store in W'msburg on Saturday!) that I paid no attention to the rings.