I tend to wear my trainers to destruction. Recently, the soles of my feet have been getting sore when walking and when wearing my cycling shoes. Part of this is a side effect of last year's chemotherapy. However, when I looked at the insoles of my shoes they were very worn.
There is a huge range of replacement insoles available. The manufacturer ones for my cycling shoes cost around £40. I can buy a new pair of the same shoe for only a small amount more! Brands like Superfeet are similarly expensive.
Recently, somebody recommended me to try these insoles. There are various different versions, for different sorts of foot problem. I bought a pair of bog standard ones and after a couple of days use, I can say they are very comfortable. At roughly £6, even if they don't work for you they haven't cost a fortune.