We were there in Kona in July and also wanted to take a helicopter to see the lava. We got to see the lava pool in the POo OO (or PooPoo OH OH thingie) (sp) mini-caldera. I just couldn't get over the price to fly in a helicopter.
We were down in Kailua-Kona by the Bubba Gump shrimp restaurant and one of those little hole-in-the-wall tour sellers was offering flights for a Circle the Island in airplane for around $150. Thought it was too good to be true, and it really wasn't. Instead of the $150 for the Circle the Island, we settled on a 1 hour flight to see the lava in that Puu Ooo(sp) vent. It only cost us $98 per person, and Dale (the pilot) was awesome! You will have to drive over to Hilo and fly out of the Hilo airport, but we thought it was well worth it. Actually, it was one of the highlights of our 2 week trip. Most of those little tour seller places are simply fronts for timeshare sales, so immediately ask if they are pushing a timeshare tour. If they are, just say no thanks and move on. There are a couple in that little area there that have no 'timeshare motive' and can simply just get you good deals on tours.
Save yourself some serious money and take the airplane. Here is the URL for Iolani Air -
http://www.iolaniair.com/
They will sell you the $98 per person for the 1 hour flight to see the lava flow and Napoopoo vent.