If it will truly generate 10-20 years at $5k/mo ($60k/year, $600k/10 years, $1.2 mil/20 years) I'd go with what I consider fair (in regards to a brick and mortar business) 5 or 6 years of actual income as a sell/buyout price... so $300-360k.
If it's truly hands-off no maintenance, I'd ask more... but personally I'd hold onto income like that if it's hands off automated. Build 5 of those and you're making 300k/year automated. Why sell?