Around here they're worth about $600 minus whatever parts & labor it would cost to make them daily user reliable. That said, they could sell for $1-$500 as is? A gas motor & decent diaphragm pump would increase the value by $200-$350 easy.
Curb appeal is important. Rust & dirty poly tanks that make them hard to look at will also make them hard to sell no matter the actual mechanical integrity.
Hope this helps