Spectro and others mentioned are actually pretty expensive as aside from fees they quote their own prices. They are convenient for micro purchases but for all out cheapness at volume you need to buy bitcoin first at an exchange like Bitstamp and then buy Dash at Poloniex. For 11 cents or so more per coin you can shapeshift as opposed to Poloniex.
If you are doing enoght volume, contact Otoh for an OTC exchange.
Excluding fees, how about Any Coin Direct and markup value? In We Sell Crypto, they're charging ~20% more than the current DASH and BTC market prices. That's way too high IMHO.