Without knowing a lot more details, this is really tough. How many zones, hot water, steam, radiant flooring, radiators? Access to a flue, proximity to an outside wall? The ultimate BTU sizing (don't go by what's there, it may be way oversized) is also important. Many of the new ones are much more efficient, and existing flues may not work.
the most efficient are modulating, condensing boilers. The more efficient, the lower the energy costs, and the bigger the rebates or tax credits, so that should play into it as well.
How do you make your domestic hot water? Is it from an indirect, an in-tank demand system, or a separate WH tank?
Your question is sort of like walking into an industrial supply and saying you need a screw without identifying the size, material, length, or material.
All houses, even with the same size, are not created equal in their heating load, so where an 80Kbtu boiler might work in one, another might need 250Kbtu...a significant difference in costs.
For a reference point, I had a top-of-the-line condensing boiler and large indirect WH installed with three zones on an R&R, cost me $9400. I could have done it for much less, but the efficiency of this unit is pretty much at the state of the art, all SS (including the WH tank), and should last a very long time.