What is so special in the power MOSFETS IRFD120 and IRFD110?
The 100V could be a reason: high voltage MOSFET tend to be quite expensive. But I think in this case, the reason is the strange package the IRFD110 uses. This is non standard and certainly uses specific production lines.

And usually, the reason is just that: they use older technology that is more expensive to produce than newer technology. The older products then become more expensive, although they are not better speced (usually, they are worse, as you have noticed for the current handling ).

Now, if you look for a replacement MOSFET, look at the datasheet


The voltage handling (both in VGS and VDS) should be the same or above (in your case, it's below, so double check that you can do this in your circuit)
The current handling should be the same or above.
The VGs gate threshold should be similar (in your case, it slightly below, but that is less of a problem than if it's above)
The RDSon should be below
The gate charge should be in the same range (if it's a lot more, you may have to use bigger drivers).
Make sure the dissipation is appropriate.


If you check all this, I think you're clear.