Co-pays. Airlines love them. Passengers hate them. Basically, it means that after frequent fliers have plunked down hard earned miles for an upgrade, they need to pay cash money too. They’re not always such a good deal.

I get asked on at least a weekly basis, “Can’t I just use more miles and not pay?” The answer is generally no.

On the other hand, sometimes passengers can save money with a higher fare, and a lower co-pay. I just issued a United ticket to Europe this summer where the higher fare was $150 more, and the co-pay to upgrade (with the same mileage amount) was $200 less. This is seemingly a no-brainer, but then again, maybe not.

Of course there is lots of fine print with this deal. If the upgrade in question is waitlist only, fliers don’t get the difference back if the waitlist doesn’t clear. (With a co-pay fliers only pay if and when the upgrade clears.)

To complicate matters more, passengers who pay a higher fare are also higher on the waitlist. (The waitlist priority goes as follows, frequent flier status, fare paid, date added to waitlist.) So paying more does increase upgrade chances.

Plus, here’s the new wrinkle to make frequent travelers really pull their hair out — heavily discounted summer fares.

Quoting fares in a blog post is a losing proposition, because they change so frequently, but here’s an example as of today.

San Francisco to London, business class, nonrefundable super saver fare for July 20 to 27, is currently $2,829 with tax.This is a confirmed seat, with no mileage used, and actually bonus miles included for traveling business class.

Some clients wonder why business class should be so inexpensive in the summer. Part of the answer is that the airlines are not selling very many seats at this price.

But the other part of the answer is that business travel is down during the summer months (also the Christmas holidays). Airlines, realizing they won’t get as many full fares, have decided they would rather try to coax vacation travelers into paying more for comfort.

Back to the mileage upgrade deal. An upgradeable coach fare with a $900 co-pay plus 40,000 miles. ends up only saving a passenger a few hundred dollars for an upgrade that is not guaranteed. In addition, travelers lose 40,000 miles plus the mileage they would have earned. It is not necessarily a good deal.

No doubt, these fares and co-pays are a work in progress. The only thing certain — it’s worth looking at all the options. Or have a travel agent do the comparison. If upgrading is important, it is better to find out the options before ticketing a cheap fare may be no bargain and end up with no upgrade to boot.