The Ullapool ferry cost us £173 for the 5 day saver which was for 2 adults and a campervan; same cost if you take a car.
It would be cheaper to go as a foot passenger and then hire a car on the otherside or fly into Glasgow, Edinburgh/Inverness (loads of flights from Dublin). You do need a car once you are there.
If you can, camp, as wild camping is acceptable on both Lewis & Harris and there are lots of spots with toilets and water, close to some amazing beaches.
Vx