...He is wanting to live in Tagaytay area & is interested in rental costs.
Depends what he wants but try this as a starting point:
https://www.dotproperty.com.ph/condos-for-rent/cavite/tagaytay - you can tune to your requirements. It will at least give you some idea of price ranges.
I don't want to state the blinking obvious but please do go and check the property in reality if you're considering a landed house - at least a drive-by. Nothing worse than arriving at "the perfect property" to be greeted by your neighbour's five guard dogs, all barking their heads off!

(Well, actually living there would be worse!)
Scan the area for large karaoke machines, motorbike traffic, funky roosters strutting their stuff, and localized burning of goodness knows what...sadly that rules out a lot of the Philippines...

I tend to stay in condos, and go as high (level-wise) as I can!

Good luck!