Forum member asmi had an impedance controlled board made by OurPCB (which is just an alias of WellPCB). They also included a report on Z0.
But they don't use FR408, and the Shengyi materials mostly will have irregular Er at higher frequency.
They do, I believe, offer Rogers material, you can ask them about this. Those RF materials should be at least better than FR408.
Not just me - darrel also posted his experience in my thread (he ordered 8 layer boards), and he seemed happy with the result. He also mentioned that he requested Iteq IT180 material which has very good frequency properties according to
datasheet. They quoted me $140 for ten 10x10 cm boards 4 layers 0.1/0.1 mm traces, 0.2 mm min drills, ENIG and controlled impedance (+$30 shipping via DHL), I ended up ordering 6 layer boards with same specs for a little over $200 (shipping is again $30 extra).
So while obviously not "$5 for 10" level of pricing, but I still think it's fairly cheap for the specs they have (darrel mentioned that he paid a bit over $300 for his order, while the same boards made in US would be $1800).