What would you plan to do with the raft?
I found an unused Sevylor Trail Boat on Craigslist for $10 - didn't need it, but figured why not? It weighs about 2 1/2 lbs and it packs to about the size of a rain jacket. It's insanely small. I haven't used it since I bought it 2 yrs ago.
I usually use a Sevylor Caravelle 3 man raft for backpacking/ fishing. I prefer to troll while fishing. I don't believe any of the lightweight rafts have oarlocks so trolling would be difficult. You'd also need a leash to keep the paddle from getting away while reeling up a fish. I sometimes use an Intex Explorer 200 for trips where I want to go lighter but still want to troll. It weighs 4 lbs without oars.
I haven't bikepacked yet, but I've done many road tours. If you just want something to float on, you might try your sleeping pad or even a 99 cent air mattress from the sporting goods store.
Did you find this site in your search?
http://flyweightdesigns.com/default.aspxLooks like another option - maybe a little less money than a Alpacka.