Everybody wants and needs to keep the cost down.
However............cheapest is rarely best as these two seem to be mutually exclusive.
Best options might be to join a club (non-profit) and exchange work for reduced rates then a flight school which is by definition a business (for profit).
Glider clubs vs commercial glider sites are a prime example.
You might want to consider doing a accelerated course (3-4 weeks) that might be offered by clubs or schools during the summer months.
So use teh winter to get your theory exames completed and use the summer (weeks) to bang the hours on a reduced cost accelerated course.