Go for the low-hanging fruit. Translation: You might not need to run out and buy a super-expensive and fancy new watch.
Did your current watch come with a strap that is sitting forgotten and unloved in a drawer? Grab it.
Is your current watch heart-rate enabled but you didn’t buy the strap at the time because you didn’t think you needed it? Get the strap that goes with your watch.
If you’re ready to upgrade your whole system, check out the price of the watch you want. Sometimes the stand-alone strap is pricey enough that it makes more sense to go ahead and upgrade.
Don’t have a watch because you run with an iPhone? Look at the available GPS apps like Strava, MapMyRun and Runkeeper. All will sync with different heart rate straps, so check prices and pick the combo that works with your budget.