Costs are going up which means increased competition. Increased competition will mean your ads are running against more competitors. Reason for fall in click-throughs could be under-optimized ad's losing out to competition's ads.
As for conversions. Looking at the table plan site I feel overwhelmed. If this is where the ad's are taking users I think it is fairly obvious why sales are dropping off.
I don't think Adwords can be blamed for the profitability of a product.
This isn't to say Adwords isn't declining in value for site owners. To make this claim though you would need information from lots of sectors from lots of site owners.