I have an old Moonstone brand light sleeping bag, cant remember what it was rated for, but then I tend to sleep "cooler" than others, so as I camp in summer generally, its been ok.

One point about buying a sleeping bag / cost, is that I have always found that a reasonably priced lightish one does me well, and if it is cool at night, I always have a fleece and long pants anyway, so if need be I will wear all the extra stuff I have anyway on a trip and that generally covers it for me to be comfortable.
What Im getting at is that there are many reasonably priced bags that are more summer than 3 season bags, but can be both small in packing, and also be supplmented for warmth with what we usually have with us anyway (take some warmer wool socks and a light toque if needed also) so one doesnt have to spend a fortune for a lighter bag that is less bulky, bulky being the important factor for us on bike.

It has been very rare for me to be bike camping when it gets anywhere near freezing, but of course if thats an issue then you would want to rethink what bag you have.