The Happy River can be done for less than $300/person, even without pilot friends and their boats (which could bring price down even more) by judicious use of walking and timing. For example, assuming that the mail plane from Skwentna to Anchorage leaves on Tuesdays:
1. On Thursday, charter a wheel plane (like a 206) from Willow with 4 people going lean and light to land at Puntilla Lake airstrip (i.e. Rainy Pass Lodge) early in AM. Here leave a cache of food and shelter for rest of trip to Skwentna so you can hike upstream and run the best bit of the Happy. Follow awesome horse trails to above Sheep Lake. Ask Shane [one of the stars of the reality show "R 5 Sons"] at Rainy Pass Lodge for trail beta. Looks to be 15-16 miles up to above Sheep Lake; carry just one overnight worth of food/gear. Maybe sleep in cabin/wall tent at Sheep Lake? but watch out for bear there.
2. Friday, float Happy down to crystal clear Indian Creek -- my favorite part -- lots of granite boulder gardens and beautiful views of mountains. River's a beautiful color as it's not mucked up by Moose Creek's glacier melt yet. Follow good horse trail back to Rainy Pass Lodge to get remainder of gear, hike back to river and camp [or pay $100/person(!) to sleep at lodge].
3. Saturday, paddle and play down to end of Happy (side trips en route....Glacier Creek?)
4. Sunday, paddle down to Hayes River (side trips en route) on mighty Skwentna. Moves pretty darn fast -- we made it from Happy to Old Man Cr in less than an hour.
5. Monday, paddle to Skwentna. Again, very fast float, if a bit chilly and sometimes sleepy -- wake yourself up by standing in the boat and/or hitting eddy lines and corner stacks.
6. Tuesday, catch mail plane (check on times and days) to Anchorage, or have pilot friend pick you up in boat or plane.