A high-level philosophy:

If you care more about quantity of travel than quality - especially if it's domestic US travel - mile valuations in the 1.3-1.5 range are much more typical.

In your case, it's important to separate signup bonuses from daily spend. Example:

If you value UA miles at 1.5 cents and the offer is 50k miles for $2000 spend, you're earning $780 for $2000 spend, or a return of 39%.

Going beyond that initial spend, you'll earn 1 UA mile/$, or 1.5% back. At that point, just spend on a 2% cashback card.