I think the issue is product availability. Here in Canada we had a shortage in pet stores for months, I couldn't find it or order it from anywhere. One store got a few containers, but they sold out quickly and they couldn't get anymore back. Online prices are probably going up because of this shortage.. perhaps Solid Gold isn't making it anymore?
I used to rotate Solid Gold, Nupro Gold and Kelp with my dogs... due to the shortage I decided to craft my own supplement. Holistic Blend's Seagreens is a good replacement for Solid Gold is you need to switch.