I have heard the dish soap remedy before.  You have to really soak the plants good with the soapy water. Use a strong mixture of soap.  I have also heard of people using tobacco in water to spray on plants to kill mites.

The idea of the oil is to spray the undersides of the leaf to kill the eggs.  Usually this is started in the spring.  You can use any "superior-type self-emulsifying petroleum oil" according to Reader's Digest.

The recommended pesticides are Kelthane or Tedion at two week intervals during the summer.

This website has oil at a cheaper price than you quoted.  Much cheaper.

http://homeharvest.com/insectspraysoilbased.htm

http://www.bestnest.com/bestnest/RTProduct.asp?SKU=BOP-214

The ingredient listed is just "petroleum oil", so you can possibly find an even cheaper substitute in a hardware or auto parts store, but these are $9 a quart and you mix them with water at the rate of 2.5 to 5 tablespoons of oil per gallon of water.

Or use Murphy's Oil Soap in a strong solution.

Spider Mites hate humid conditions, so frequently spraying your plants to keep the leaves wet will help.  Concentrate on the undersides of the leaves whatever you do as that is where the eggs are.