If you are not good at bargaining, it is OK to pay a little more. The amount usually is not too high, and is definitely worth it. Unlike many other tourist spots around the world, you will not end up buying a fake artifact. The good news is, the priests in the temples are almost always sincere. Unlike some very famous temples in India, they don't demand money from visitors.
Donations are always welcome.
That doesn't mean that the priest will start abusing you if you don't donate money. Along with bargaining, tipping is also something that will make your life a lot easier in the city. You can tip the room-boy in your hotel, and he will bring the world into your room. Most important of all, in the small temples, you might usually find a care-taker, who will narrate the story of the entire temple to you and let you take as many photographs of the temple's treasures. Sometimes, they even advise you of the most popular photographic locations in the temples.