I recommend buying a bag rated five to ten degrees below the chilliest temperatures you anticipate. Don't think strictly in terms of price. True, most synthetic bags cost less than down, because their innards are about half as expensive as the feathery fluff. But a down sack will also outlast its synthetic peer by five years or more. Price aside, each fill material has its merits.
Synthetic fills retain much of their insulating ability when drenched.
A synthetic-fill bag is also easy to clean, hypoallergenic, resistant to rot and mildew, and much easier to dry out if your kayak tips over and dunks it. The most popular choice is Polarguard 3D, a fill Hoechst Celanese introduced two years ago. As good as the new synthetics are, down is still unbeatable for its combination of light weight, compressibility, durability, and warmth per ounce. True, down is next to useless when wet and almost impossible to dry in the field. But during 25 years of camping in the notoriously damp Pacific Northwest, I've never had a down bag go completely limp on me.