I apologize for the poor wording. You're correct; all things being equal, it's better for you if there's a higher ratio of PQD to dollars spent. The issue is that there are certain taxes and fees that apply regardless of the cost of the ticket -- the $5.60 security fee each way, plus up to $18 RT in Passenger Facility Charges, depending upon whether or not you connect. On a cheap ticket -- say, $79 or $99 one way -- these fees make up a high percentage of the ticket cost, and they don't earn PQD.

So, the purpose of my post was to say that if you travel on super-cheap domestic tickets, you might have to spend even more to reach Silver than if you travel on more expensive tickets.

There are threads on how valuable it is to reach each status level. If you find that a lot of your travel is on foreign carriers, UA Silver isn't particularly valuable, but UA Gold might be -- especially if you're flying in economy on those carriers. UA Gold gives you Star Alliance Gold (*G), which gives you an extra checked bag, priority security and boarding, and lounge access, even when you're seated in Economy.

So, while the rest of the MileagePlus Explorer card benefits aren't as good as the Platinum AmEx, it might be worth moving $25K of spending to that card just for the chance to reach *G.