Welcome to Scubaboard and PPD.
With regards to scuba training, look for quality, not price. Whilst there are some regional variances in cost around the Philippines. However, within a specific area any variance in course cost is likely to reflect the quality of service offered.
Consider the following criteria, as a starter, when researching course provision:
1) The number of students per class/instuctor. (8 is the max, 4 is reasonable)
2) The quality and quantity of dive site/s visited during training. (boat dives or shore dives...coral reef or bare sand?)
3) The experience of the instructor (how many years diving, where they dived, how well qualified they are, how many students they've taught)
4) The quality of rental equipment used. Is it well maintained and fully functions... or threadbare, improvised repairs and often faulty.
5) The quality of compressor, which provides your breathing gas (is it tested/verified to international standards?)
6) The quality and type of dive boat used, and whether is is equipped for safety (emergency O2, lifejackets, first aid kit etc)
7) The duration of the course.. is it crammed in..or leisurely, with opportunity to practice and learn at a sensible rate? 3 days is low...5 days is high.
8) Whether the dive shop/instructor follow, or exceed, agency training standards.. or whether they break them or just provide the minimum levels.