A good phone case to mount on your handlebars might cost $50 (you can get cheaper, but you'd want waterproof in case you get caught by changing weather), lets you see the number whenever you look down.

Garmin's 130 is on sale for $150. It's waterproof, the battery is good for a century ride, it has GPS but not navigation, which means you can look at a map of where you rode, and see things like whether a high HR reading was during an uphill effort. I don't know whether it works with your strap though. Some of their watches will do what you want and do work with your strap. You put the watch on your handlebars. These are in the $300 range.

Someone else can comment on other options.