There are a bunch of possible suggestions. I ran a Trijicon RMR Type 2 (the 3.25MOA RM06 model) on a Dueck Defense mount on my 2.0 compact for a while before I got my 2.0 COREs, and I was pretty happy. The mount has built-in backup sights and is really well made. It needed some fitting but was rock solid.
My only complaint was the set screws. One is located right above the plate for the striker block plunger spring. I didn't check to see if it would impede the function of the block if tightened down... I just took that set screw out and only used the other one.
As for performance the RMR/DD mount worked great. I have used and have partners who use DeltaPoint Pros (a good one is good, and a bad one is bad), RMRs, a couple Aimpoint ACROs, and a smattering of other optics.
If I were on a budget and wanted to experiment with optics, I'd go with the aforementioned DD mount (so no milling necessary) and a Holosun 407C. It's inexpensive and a nice durable optic. If you want to go with a really good setup, I personally think direct milling is the way to go (vs. a factory optic gun) and I'm very impressed with my RMRs. I've owned three and I have two Type 2s now. They all work great.
Dot size is subjective, It seems most people prefer the 3.25 (or smaller dots) but some prefer the largest dots due to astigmatism, specific close-ranger needs, etc. I have both 3.25 and 6.5MOA models and can't decide which I like more.
There is a learning curve that is shortened significantly by proper training, so another aspect to the cost is the dedication to practice and skill development. I looked at pistol optics for a long time before I jumped in the deep end. For many years I thought acquiring the red dot was simply too slow. It took very little time with the correct technique to learn I was very wrong. If you don't have a way to get quality training from someone who knows what they are doing, you'll have to put more time in, but I have a few YouTube videos I could recommend.
