It's actually pretty cheap to buy the necessary logic on chips, and it doesn't have to be a huge ALU either.
http://cpuville.com/ALU.htm gives you an idea of what it takes for an ALU fully integrated to a computer. Implementing an ALU by itself would mostly require some redesign on the part of the Input and Output sections, and maybe a reduced complexity/opcode section (maybe only a few instructions and a few switches for operations without multiplication/division)