It's absolutely no surprise that they're not factoring ad-revenue in. If you have a market with people who have the intent to purchase (a la Zillow) as well as the means, that traffic is worth about as much as commercial insurance leads or recruiter leads for a good candidate. The estimate made by the grand-father poster is right in terms of how startups are traditionally valued; however, I'd argue that you're not a traditional SaaS (that tends to see explosive growth).
Grow it like a small business or find someone who's a professional real estate agent with extensive experience in the online markets, and figure out an edge. Whether it's selling insurance affiliate services or something else along the lines of that, what you have right now is a very valuable entity that's being severely under-capitalized.
The fact that you have paying users is indicative that youve generated trust and deliver value. Right now you're in a powerful position. I'm assuming most of that affiliate marketing is for creditkarma credit checks/mortgage prequalification leads. I'm not sure what your numbers are in terms of conversions but I can tell you definitively with the traffic youre pushing you're at the point where you can call up those agencies and threaten to take your business elsewhere unless they make an offer to increase your rate. If you're going through an affiliate marketing program, they're taking a cut between you and the lender. You have the power in that relationship (that is, you leave and he loses all his revenue without any recourse; if he leaves you, you can just sign up with someone else who'll happily take your leads). Alternatively, I've seen deals go through where mid-sized regional banks offer a massive rate for exclusivity.
Those numbers are great especially for mobile. If you do sell it, sell a chunk of it (~30%, no more than 49%, you want to retain ownership on this trust me. He might want majority, in which case the middle-ground is to structure the deal with buy-out terms that are amenable to both parties, allowing you to essentially defer the large decision of selling a potentially profitable, under-utilized business until you see how it develops) for _significantly_ more than that 270k-350k estimate. The reason Zillow/Redfin/etc are making an offer right now (whatever they're offering, ask for 3x) is because they know how valuable your traffic is and are aware that you're undercapitalizing that traffic. If you find the right person who has experience in that market and offer him that 30% ownership with a huge incentive bonus for any revenue growth that's beyond the estimated growth curve.
Think about it this way: 250k is the price of a moderately decent house in a non-major city. If a real estate agent manages to close 20 deals over the course of 3 years at a little over 6 houses a year at 250k, he has already passed your one-time revenue cash out at 250k. I can safely say that at least 20 agents are making their 6% rip a week because of your site with those numbers.
Those SEO numbers are insanely good, especially since mobile is so important in housing sales (e.g., 100k will get you a fantastic app that someone can turn on when they're driving through a neighborhood they like, and you can sell those listings to agents for premium access for 200-300/mo. Compare that to reddit where mobile-aware doesn't add much to the value proposition).
Email me if you want to chat more. Contact information is in my profile.