Month one you’d have $187.50 in interest but average for the year would be $186/mo. 

My average equity capture is $43k at the buy for a vanilla house. 10 years or so of cash-flow for dividend buyers. 

Big thing out here is area know what your buying and average rents because you can buy in one AREA of say NE and the highest rent you can obtain is $1500 but 6 blocks away in NE you can charge $500-$600 more - so location is key.

Here in ABQ, cap rates on multifamily are anywhere between 5-8% on average, obviously depending on where you buy. 

Your GMI is typically averaged out over two years.

The lowest BAH (allowance military members receive for housing) is 1365/month with the average around 1600/month.

I am fairly confident I can rent this out on AirBnb for around $120/night on weekends during warm months and $80/night midweek and get a fairly decent income from it - I'd estimate conservatively being able to net $1,500/month on average from May - September and $800/month October through April (would love to find monthly tenants, but acknowledge that won't be at 100% capacity during off months).I put in an offer for $40k, which is what I feel like the house is worth in its condition and location - there are a lot of houses in similar condition but with more square footage (this guy is just 650 sq ft) that have sold in the $20k-60k range in the last year.

I don't know if last year or month, but it gives you an idea of why you might do well to be cautious buying now in Austin...or why you mght follow the crowd because they expect above average appreciation.  

But I'm wondering what's the average mortgage rate nowadays for a 30yr loan with at least above 700 credits? 

I just peeked at your sales prices and they aren't all that great -- they look to be half of the nation's average.