I had a problem with an older monitor a few years ago, and sent it to AgExpress Electronics.  They repaired it, replaced all of the light bulb covers, put a new cable and end on it, and I think the total was less than $75.  I had it back in a week including shipping time.  If you're happy with the monitor you have, you might want to at least give them a call.  A new population monitor is going to be pricey, especially since you'll have to change out your row unit sensors.

HTH,

Mike