Is it even possible to make a list of spotting scopes that have MRAD/MIL or MOA reticles that cost under $500 new?

You can get close:  

https://www.opticsplanet.com/bushnell-legend-t-series-spotting-scope.html

https://www.opticsplanet.com/vortex-recon-15x50-r-t-tactical-scope-rt155.html

Find a coupon or chat them and see what you can get.

Bushnell Legend is the only one I’d use.  You need magnification to use the reticle correctly for better precision.  It helps that Bushnell has a high level of quality and they offer a lot of value for the money.  
For anything less than that, there’s really nothing.  Or at least nothing of note that’s worth spending the money.  
The next step up from the Bushnell T-Series would be the Vortex Viper HD 20-60x85 with the optional 33.5x ranging reticle.  The glass on the two are similar, but the Viper has the much larger 85mm objective lens which lends a lot to performance.  After that, the next steps up are going to be well over $1k and you’d be looking at the Leupold Mark4 or Bushnell Elite Tactical LMSS2.  

I realize you may have a budget, but with spotters you need to get the best scope you can afford.  It will pay dividends.

I have a Bushnell spotting scope with MIL reticle for sale - like new, used 2x for sale. PERFECT condition

this is it -

I had not one, not two, but three of the Bushnell Legend T series spotting scopes. All of them were garbage.

Spotter #1: The zoom ring broke and the scope was stuck on 45X. I had used it maybe 5 times over 6 months. They told me through email it was a known problem with the “glue” of the zoom rings sometimes coming apart. I spent $27 on shipping and sent it in for warranty work. They sent me a brand new one.

Spotter #2: The glass looked “speckled”. Literally looked like a tiny fire cracker went off inside. There were hundreds of tiny black dots all over the glass. The Mil reticle also appeared to be crooked. Like the the vertical line was at 1 and 7 instead of 12 and 6. Once again spent $27ish on shipping to send it back. They sent me another brand new one.

Spotter #3: Mil reticle was now straight, but the tiny black dots were still all over the glass. Cleaned both front and rear glass, confirmed black spots were on the inside. I had 3 other shooters at a local competition look through it to confirm I wasn’t crazy. Everyone of them could clearly see the black spots except for an 80 year old who admitted he had terrible eye sight. I said “great, what will you give me for it?” And I sold it to him at about a 30% discount. I was done spending money to ship them back just to get another with problems.

Having said that, I have the old Bushnell Legend Spotting scope, the original black one that looks right side up with no reticle. And it’s been 100% for over a decade. The Bushnell T series was meant to replace it. Thank goodness I hadn’t sold it yet, because that’s what I went back to.