FWIW, I sent a 10x10cm PCB to PCBWay on 11-Jan-2017. They charged $10 for the PCBs (several of the Chinese sites have recently lowered their prices for larger boards, or have "specials" - I'm not complaining!) (it's especially nice to have the max "free EAGLE" PCB come in at the minimum price!) They charged $21 for shipping, shipped using DHL, and I had the boards in my hand on 16-Jan-2017. They're lovely and work fine (except for my own mistakes), and I was very impressed with the turnaround time; I was expecting the old 2-4 weeks... (Oh: they let me use $2 from a $5 "new customer" coupon as well!)
https://www.flickr.com/photos/58843278@N00/31450347183/in/album-72157622049680333/In contrast, I sent two smaller boards (one 5x5, one 5x10) to Seeed on 16-feb-2017. The PCBs were $10 each, and they charged $15.30 for shipping. They claim to have shipped on 24-feb, they used the normal "postal service", and I'm still waiting for them. (This is about what I expect, and I'm not complaining. But PCBWay was a lot more impressive - I had boards in my hand in less time than Seeed took to make them.)
I consider both shipping charges to be "reasonable." I'm pretty sure that they're in-line or better than what it would cost me to ship in the other direction. One "problem" is that the low-price "Hong Kong Post" package has a weight limit (100g?), and it doesn't take much FR4 to exceed that limit and get you into the realm of higher prices. My 10x10 PCBs are about 30g each. (I guess this might be a reason to order "less than the cheapest amount" (5 vs 10) of a PCB? I haven't tried that yet.) (Hmm. Itead charged me $3.90 to ship 10 5x5 boards, back in 2013.)
So I've used Dorkbot (2010!), Itead, OSHPark (several small boards), PCBWay, and now Seeed for one board each, and I've been happy with all of them. OSHPark is probably my favorite, because three boards is usually plenty, and a lot of things I want to do with PCBs are very small, but they get very expensive when your boards get bigger :-( I haven't used any of them "a lot." Yet.
I intentionally design my PCBs very conservatively, though (well, there were those SiLego GP12 breakouts... Small size, small risk!) It's nice to be rewarded for such conservative practices, by being able to pay lower prices. Now if only I could find someone really cheap to do those 30cm PCBs with the 5mm design rules (for the Wimshurst, you know!) :-)