From looking at the website’s advertised interest rate (which was 2.75%) my impression was they were just looking at the date the property was purchased and estimating a mortgage payment based on the market rate at that time, 2.87% per Fannie Mae. 

We have the opportunity to advertise on one but have no idea on a fair price.

What is best way to advertise short term rentals?

In all reality since you continued to advertise the property and bookings came through, you need to honor those bookings.

There are NO national HML/PML  not one company funds in all 50 states many will do the majority but NONE of them are nationwide even though many advertise that but its a little puffery by some of the legit ones .. but fake ones keep in mind NO one funds in all states.

What is best way to advertise short term rentals?

I even checked in Cross Property Search to make sure these aren't being advertised in a different category. 

Trying to understand how investors assess whether something is truly a good deal as its being advertised. 

Or are you just advertising this development ?

Get a simple system for advertising, vetting potential tenants, and collecting rent together, and put the house up for rent.5.