Need some advise.

Now i stay in Bangkok but next month i like to visit Manila for perhaps a 8 weeks trip.After my philipine trip will go back to Thailand.

The travel agency in bkk told me it most buy a return ticket that i need to purchase with them or have a onward ticket leaving Manila.a return ticket bkk mnl mnl bkk valid for 2 months period costs 16000 bht.That is a high fee in comparison with return ticket valid for only 2 weeks period that ticket cost only 7000 but since i intend stay longer the ticket get priced up lots.i would be even fine to buy a one way ticket Bkk Mnl it costs only 5000 baht but i not sure it will work with immigration and how i can get a onward ticket reservation proof to show up with anyway just look for a better deal then the 16,000 which i believe is an unfair price.Any advice more then welcome

Buy a throw away ticket to Kota Kinabalu or such thing else.

Go to the website of Air Asia or another regional airline and work it out for yourself. If you want to leave your stay in PI open ended follow advice in #2.

so is it possible to buy a throw away ticket for onward flight from some airline without using that ticket and without loosing that money for that ticket?

What Philippine Visa will you be traveling on ? A number of foreign nationals are allowed a 30 day visa on arrival. I travel quite regularly to Manila on business and of course always have a return ticket, but I have never been asked by immigration at NAIA to see it, every time the question is the same " how long will you be staying in the Phils ?" When answering three days I am stamped through. However your answer of 56 days may make heads turn

I am an european passport holder yes i will receive a 30 days entry stamp up on arrival in Manila.but i been told that even at bangkok airport at the check in counter for my manila flight there they will ask me for my onward ticket out of phils.and possibly at the manila airport will ask a onward tiket also.but biggest obstacle for show onward ticket is at bkk airport during check in

You mention you will fly back to Thailand after the trip and that PR immigration require a return/onward ticket. Simplest thing would be to buy a return ticket? What airfares have seen online with Cebu Pacific or Air Asia? 7000 or 8000 baht return should be doable if you are a bit flexible.

so is it possible to buy a throw away ticket for onward flight from some airline without using that ticket and without loosing that money for that ticket?

The sense of throw away is to don't care about the small amount.

What Philippine Visa will you be traveling on ? A number of foreign nationals are allowed a 30 day visa on arrival. I travel quite regularly to Manila on business and of course always have a return ticket, but I have never been asked by immigration at NAIA to see it, every time the question is the same " how long will you be staying in the Phils ?" When answering three days I am stamped through. However your answer of 56 days may make heads turn

It is possible to extend the 30 days to 59 days at every immigration office and also directly at the airport. There is a fee for such a extension.