I started 20 years ago myself, in my area the prices I was charging for a 3 bed semi was around the £3.50 mark.
Now the same house would be closer to £8.50.
I would say that wherever you are, don't drop below 75p for a standard size window.
I know it rather depends on the area in which you work, but even for the smallest of houses I would not price below £6.00.
Ian