it can be a toss up but generally, when we try to stick to one or two airlines in our travels, we sometimes do pay extra to get miles. An example of this is when someone you know from work who has no idea what goes on in FT land gets all excited about how they found a cheap flight on United or Continental last month for something, and then another time they got a cheap flight on Southwest, and then later, they found one on jetBlue or AA or DL... They do not earn miles nor care as much about how to most efficiently gain them for future use (sooner rather than later) No, instead, they go with the "let me get the best rate today only" philosophy.
On the other hand, had that same person been a member of United MP, for example, and decided to try to build mileage for later use on something they DO want, all of those flights might have cost a couple bucks more, but the end result could be worth it!
I stared out long ago trying to gain miles on UA and I knew 25,000 was a free domestic ticket. That amount will get you on a plane that could otherwise cost $200 Rt AND it will get you on a plane from BOS-DEN-ASPEN which could cost upwards of $650 RT! I saved my miles for such a trip and when the time came, I had enough faster than the other guys who wanted to ski there. Some of them ponied up the cash. OUCH!
(this was in my early days of gaining miles before I even joined any forums. I have since gained and spent many more mile with UA and its partners)
It depends on your needs, future plans and rate of travel, plus what you think is worth paying for. In my experience, I think NWA generally gives a decent "conversion" rate for miles and $... And if you get into some of the jams I have been in recently with that airline, a $200 voucher can convert to 25k miles, a $50 voucher can convert to 4-5k miles, and having to mail back a mistake ticket on printed stock that costs you $15 in overnight postage will yeild around 2,000 miles if you ask CS nicely for it. Working in reverse, these amounts are pretty much what I consider benchmarks on mile-value--roughly.
MM