(2-part post):
There are obviously three main ways to do things:
1. You travel to Glasgow and then you both fly from there ;
2. Your mother travels to Manchester and you both fly from there ;
3. You travel independently and meet up in the Isle of Man.
(I know that there are ferries as well, but I think that you'll find it easier, and possibly cheaper, to fly)
So, I'll just quote fares for one person on each route and you can combine them as appropriate. The cheapest air fares are usually obtained by travelling mid-week so, if possible, I suggest travelling out on a Thursday and back on a Tuesday. You should also avoid travelling close to either of the bank holidays in May. (Also remember that the he TT races start on the 26th).
EuroManx fly from Manchester to the Isle of Man. Flying out at 1610 on Thursday 17/05 and back at 1220 on Tuesday 22/05 will cost you a total of �88 per person (including taxes, charges and fuel supplement). There are actually 4 flights daily, in each direction, so you wouldn't have to stick to those times (although travelling at different times might cost you slightly more).
http://www.euromanx.com/
FlyBe also operate on that route, with 5 flights in each direction daily. At certain times, they're cheaper than EuroManx (but not always). Using the same dates as above, with an outbound flight at 1450 and a return flight at 1340, the fully inclusive air fare is �73.98 per person.
http://www.flybe.com
The only direct flights from Glasgow, to the Isle of Man, are with LoganAir (who operate under the British Airways banner). There's just one flight, each way, daily. Using the same dates as above, with an outbound flight at 1110 and a return flight at 1230, the total inclusive fare is �135.30.
17:17 Thu 29th Mar 2007