Start by looking at past reported sales of similar domains at DNJournal and other sites that archive past reported domain sales.
If the domain is making regular income via type-in traffic you can build an argument for a baseline value equal to a multiple of the annual type-in traffic revenue. x3? x5? x10 revenue? That depends on the likelihood that whatever "generic thing/good/service" people are searching for will remain popular and commercially viable.
As always, one of the best guage of a domain's aftermarket value is the number and quality of past "meaningful inquiries" = concrete offers to buy.
When it comes to selling one can always start high and see where/how the market responds. You can always ask for $100K or $1million . . or more . . and see if the market ignores you, laughs, or counters with $500K. Of course, the person/party countering with $500K may just be "pulling your leg" (joking) . . because they think you're (whatever).