Agree with all the above comments - it depends… For me, weather is also a huge issue. I wouldn’t leave California for - almost anything! But I don’t live in the Bay Area or near San Diego - both are way too expensive… I live in the suburbs of Sacramento - large city, check.
There are several GOOD trainers within an hour’s drive (and many within 30 minutes drive). For top quality international riders - the Bay Area isn’t too far away, and we even have some real up-and-comers here in the suburbs of Sacramento (as in international competitors), as well as USDF faculty, USEF judges, etc - all of whom offer training and lessons. And top clinicians come into the area - including Olympians. Trainers - check.
Moderate cost - again, that is all relative. There are plenty of good barns in the area that are in the $500 to $750 range for board and turnout (turnout is necessary to my Pony’s well being). Training ranges from $600 to $850, single lessons from $75 to $100. Clinics are more, all depends on the clinician. CA is not as expensive as people think - IF you avoid the Bay Area and So-Cal.
Weather is relatively doable - no hurricanes or major winter storms. No snow or ice! Summers are hot - but a dry heat, very different from the humidity of the East Coast, and at least in Sacramento area, it cools off in the evening, so if you can ride evenings or mornings - it is very pleasant.
You really have to define reasonable costs, and what you consider a city (and how close you need to be)… Also - what kind of training do you want? And is show access important to you? Weather? Do you work, and if so, what industry (aka do you need to find a job in a specific industry)?