"Make me an offer via text message." Uhmmmm help?

[Ryan Dossey]

I got a text from our Google voice account. "Hi my name is ______ I have a duplex I'm looking to sell. It is located at _________. It is rented for 600x2. One tenant has been there 5 years other is on a fresh lease. It needs no work." 

My answer: "Oh wow! Thank you so much _______ for reaching out to us. We are always looking for multi family properties. What are you asking for the property? "

Reply: "Make me an offer."

From looking up tax records it is owned by an LLC so I am assuming an investor. Being a duplex none of the usual websites are even giving me estimates of value. I assume just run comps and offer 65-70% and let him know we will adjust for repairs upon inspection? Or do you go off of the monthly income and our minimum cashflow and work backwards?