I can't compare with Goa as I havn't been there.
The mozzie pills ?..
Personally I don't use them in Sri Lanka.. The incidence of Malaria is very low so I take the calculated risk and avoid anti,malarials.. I do use lots of repellent, sleep under a net, and usually burn a coil in my room during the evening. I also try to get a coil burning under my table when eating outside during the evening.
If your staying in a big , air-con hotel then the rom coil and net may not apply to you but I use small guest houses with no air-con.
With the anti-malarials, thats only my personal feelings.
Overall, I'd estimate , that most visitors do use them..
Bentota I don't know. It's a tourist spot which I try to avoid. For food,, depends entirely on how and where you want to eat.
If you use a hotel or tourist restaurant, then a decent lunch will start at about 350 rupees upwards. Evening dinner at about 550 to 600 upwards.
Drinks,, a cold beer in the hotels will probably cost you about 200 rupees, perhaps a little more. Local gin and tonic, ( generous measures) about the same.
If you eat in the small , local cafes, usually called " hotels" although they don't have rooms to let, .. my usual lunch is a plate of short eats. ( Various pastries with a combination of fillings, fish, meat and vegetable, usually curried, some quite hot and fantastic.
Lunch for two , with a cold soft drink / water or tea .. comes to about 150 rupees.
For evening dinner in a similar place, an excellent rice and curry for about 100 rupees. A cold beer at the local price, about 100 rupees.
Don't forget, when you're comparing with Goa... we're talking Sri Lankan rupees here, not Indian and thats a different exchange rate.