If a boiler is setup to only run when it needs to heat the indirect, it's efficiency is quite good. This also depends somewhat on the burner design. Similar to an electric WH, an indirect doesn't have a flue and tends to be well insulated so standby losses are low - some of the better ones are rated at less than 1/4-degree per hour loss. This means that the boiler may only need to run once a day after you've used a lot of water. During the non-heating season, my boiler rarely runs more than once a day and as I walk by, the temp is often sitting at ambient.
In hot-water heating only mode an indirect is pretty pathetic with high-mass boilers (40-50%, depending on the mass & insulation of the boiler, even when set up to cold-fire) but still way better than an embedded coil. See:
http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf (see table 2, right column.) To get much better than 50% requires a well insulated boiler with heat purge control on the boiler and a relatively small burner. Using the embedded tankless coil you could easily be down there in the mid-20s in summer. (At the bottom of table 2 compare 12a & 12b, the same boiler set up as tankless mode vs. indirect. )
But assuming you can get 50% efficiency with an indirect, at $3/gallon oil thats 138000/2=69000 BTUs of hot water at the output of the indirect for $3, which is 23000 BTU/$. If oil hits $4 that's 17,250 BTU/$
At 15 cents/kwh electricity and a EF 0.90 electric tank you get 3412 x 0.9=3071 BTUs at the tank output for 15 cents, which is 20,475 BTUs/$. At 12 cents that's 25,592 BTU/$, but at 20 cents 15,355 BTU/$, not such a great deal.
During the heating season the average AFUE of the boiler will be more like 65-75% efficiency (depending on the oversizing factors- see table 3 of that document), so taking a middle number of 70%, at $3 oil you'd be getting 96,600 BTU/gallon, which is 33,300 BTU/$ - a better deal than electricity for most, and at $4/gallon it would still be 24,150 BTU/$, comparable to 12-cent electricity in a standard electric tank.
Which is why plumbing an electric tank in series with the tankless coil (but
controlling the boiler only in response to space heating) is probably going to be the lowest operating cost for most people. The extra wintertime of the hot water load improves the boiler's as-used AFUE, and you can just turn it off for the summer without changing any other controls and you'll still have hot water.