I totally know where you are coming from. At this point, though, rather than pricing tickets, I would be looking at which route to take. I saw a Primitivo suggestion up there which works perfectly for 3 weeks depending. You could start in Oviedo and very resonably make your way in. If you are an agressive walker you can combine the San Salvador and Primitivo in that timespan, though again that would be aggressive. The Norte offers beautiful coastal views, though experiencing the whole thing from Irun to SDC in that timespan would be extremely agressive. I will be travelling the VdLP, new to me, in about 22 days next year, starting in Caceres rather than Sevilla because I just don't like going on a Camino and not ending up in SDC. Others, with more patience than I, take a path to enjoy a first part and maybe the next in a second year. Of course, if you'd rather do the Frances again, 3 weeks could work from Leon to Finessterre and Muxia, or from beyond Leon somewhat, maybe burgos?. I am a fast and long walker so take any suggestion from me with a grain of salt.
Now as far as watching airfares, I do too. I have learned that if I watch regularly, every once in a while there is a DRAMATIC drop in price. Literally one day a fare will be cut in half almost. If you see this, jump on it. Because the next day, it will return to normal. I caught the dramatic drop once a couple of years ago. I missed it this year becaue I thought it would last. I use kayak.com to browse available flights with my intended flight dates.