And Evercoin's fees are even higher.

I just tried placing an order and Evercoin's fees appear to be significantly lower.

Shapeshift: 1 BTC -> 14.20477011 ETH

Evercoin: 1 BTC -> 14.25967407 ETH

Changelly: 1 BTC -> 14.35434362 ETH

While Changelly appears to have the best rate they don't guarantee the rate. In my experience the actual rate is often much worse than the estimated rate (and unfortunately it's too late to cancel once you see the rate).

I was originally testing BCC to BTC, so it looks like it depends upon what you are exchanging:

Shapeshift: 1 BCC -> 0.06773499 BTC

Evercoin: 1 BCC -> 0.06682701 BTC

Maybe someone should build a Kayak for Bitcoin exchanges, that picks the cheapest exchange for each trade...

Writing the formula for you

1) pick the cheapest exchange 2) sell at expensive exchange price 3) add commission 4) add network fees

The formula is more complicated than this. Services such as shapeshift.io guarantee the price for 10 minutes, as well as sending purchased tokens almost instantaneously. To accomplish this means keeping inventory and becoming vulnerable to volatility risk and liquidity risk.

Picking a reasonable bid-ask spread is hard, and probably involves more input factors than the 4 you listed above.

That thought definitely crossed my mind...

They're all going away after cross-chain atomic swaps become THE thing.

Yes. XCAT is definitely the way forward for transactions that involve only XCAT-compatible chains. Especially as it can be implemented either in a pure P2P fashion or with trustless exchanges, resulting in zero or very low fees.