Buddy of mine owns a single engine 135 operation. Took him a year and a half to get the certificate. When he first approached the FSDO they sat him down for about 2 hours, and basically tried to talk him out of it. After he said he was still in, they were helpful but it still was incredibly difficult.

Read through 135 and look at how much harder multi engine vs single engine is from an operation standpoint. Maint cost per the regs alone are significantly higher than single engine operation. Annual W&B, 100 hour, 50 hour, etc. Direct operating cost is only part of the story. Local operation here has a Seneca II that they have to bill out at over $500/hour. I rarely see that airplane move.