I don't think the pricing on these routes are so much about competition, rather a reflection of the areas they serve.

Clearly, SNA serves a more desirable region than ONT. In addition, the people and businesses who fly this route, I'd guess, don't care that it costs $250...so from UA's perspective, "if it ain't broke, don't fix it."

SNA can command the prices it does because the alternative is LAX: rental car shuttles, longer lines, need to arrive earlier before flights, endure the 405 down to OC which is 45 minutes with NO traffic (rare occurrence).