Jastafford, you do not mention where you are so we can not estimate your diesel price.
Last weekend I put diesel in Brassbounder in Anacortes at Cap Sante fuel dock for $2.78/gal (over 100 gallon price). That is about the best price in this area and I am told it is much less than the gas station price on the highway. It is, as Bjorn mentioned, dyed, ultra low Sulphur diesel.
If you have Lehman 120's you are going to use about 6 gallons for the round trip to the available fuel dock which is, in round numbers, about $16.00 to $17.00. It would take some time to pay for a transfer tank at that rate and it is much more fun to take the boat to the fuel dock