Get a Revolut card and use it for fee free card payments, fill it up now before the currency exchange gets even worse.
We went in October and it was fantastic, we did the Northern Lights super Jeep tour, a minibus tour of the Golden Circle (both with
http://discover.is highly recommend them) and an Icelandic horse session with
http://islenskihesturinn.is which was also fantastic.
We booked flights with easyjet direct from Belfast, and sorted out own accommodation because most of the package deals had god awful looking hotels or hostels. Cost us £1200 for a week for flights and accommodation for two. Car rental is also a must I would say, it's quite spread out so being able to hop in a car and go somewhere else on a whim is great.
Food booze and shopping is very expensive, just double the prices over here and you won't be far off.
Edit - buy a DSLR camera and tripod if you don't have one already, learn how to use it and you will get some great photos. Most of the colours of the Northern Lights are lost on human eyes and using a camera is the best way to see them in their full glory.