OSH Park has a 5mil trace/spacing 4 layer option. No e-test (Got burned by that one. Always double-check your boards before assembly!) and the schedule is a bit unpredictable.
There are a few options for 6+ layers that don't seem too unreasonable. For example, Pheonix PCB does 100sqin of 6-layer board for a single design for $550. They go down to 4mil trace/spacing and 8mil drill (about $90 extra). There might be better options. At this point, for the hobbyist market, design tools start to be the barrier, not manufacturing. $500 for a bunch of prototype boards seems like peanuts compared to the price of Alitum.
There are a few options for 6+ layers that don't seem too unreasonable. For example, Pheonix PCB does 100sqin of 6-layer board for a single design for $550. They go down to 4mil trace/spacing and 8mil drill (about $90 extra). There might be better options. At this point, for the hobbyist market, design tools start to be the barrier, not manufacturing. $500 for a bunch of prototype boards seems like peanuts compared to the price of Alitum.