My 2 cents
I don"t think you can get a new population monitor for $800 unless you are just talking about the monitor box itself.
The seed sensors are close to $100 each themselves.13 equals $1300 for your 13 row. Then you need wire harness, a speed sensor and the monitor itself.
Seed monitors are very touchy and buying used may be an issue unless you find a seller that will be honest about condition.