Welcome to the forum, Don. Yes, it is possible to build a home of that size within your budget in the province, however I have some questions first, and please do not misconstrue them, I just want to make sure you're not stepping into this completely blind and don't get separated prematurely from your assets. Your introductory profile was not specific in this regard.
Have you been to the Philippines in person before?
Have you been to the building site location? In different times of year?
This one may sound silly, but have you actually met your fiance in person and spent extended time together?
You would be surprised how many foreigners meet someone online, fall in love, and begin making big plans before being able to answer yes to any of those questions.
As to the land, the cost will of course depend on the site, any nice features, access from the road, utility access, any potential issues that need mitigating, title, etc. It's hard to give an estimate without knowing more about it. One thing to keep in mind is that real estate in the Philippines, especially in provincial areas lacks transparency. There is no Zillow, or other comparison/valuation sites. Title is often an issue and can be disputed by family members.
Keep in mind also, that as a foreigner, you cannot own the land. There are some exceptions (long-term eases, etc) but they are fraught with risk. So plan on it being hers. Which means you will have little claim to what you build on it, especially in a rural barangay, where she and her family will hold sway in the event of a dispute.
Hence my questions. There is an old saying around here not to invest more than you can afford to lose. Also to be at least one island away from your in-laws, haha. There are good reasons for these sayings. Many have lost everything or find the in-laws visiting regularly and asking for support. Again, not saying that's your situation, just making sure.
Another simple thing to consider is what you will do there in a rural barangay? Many have poor or nonexistent cell service, no decent shops or restaurants nearby, not even a coffee shop. Of course that may not be important to you but you also have to give it some thought, as well as consider your personal safety if the area is remote.
Assuming you have spent considerable time with your woman in person, the relationship with her and her family is solid, and you fully understand what you're getting into, then the project can move forward. But not until you are there, have looked around a bit first, and can manage it. Also - is it where you really want to be? Province life is not for everyone.
I built a smaller province home on property owned by my girlfriend that turned out really well. I had been there several times before - it's on an overlooking site, with views of mountains and ocean. But the year after we built we found better schools for the boys on another island nearby, in the city we live in now, so it is currently relegated to weekends and summer use.
As for actual building costs, it can be very reasonable. I would plan on at least 25,000 Php per sqm. I spent 50% more than that because our lot is steep and required extensive foundation work and retaining walls, and I upgraded the roof, windows and doors for the climate and typhoon resistance. It was a really fun project and it is a joy to spend time there (so peaceful), but I wouldn't have considered it until we had several years together first.
Please let us know more about your situation, and I'm sure that others will be jumping in with their experiences.