Most people I know buy second hand TIs which aren't expensive. They are around the $20-50 (GBP conversion) mark here in the UK.

The Pi is not exactly a good proposition for a calculator either based on form factor and power usage. Also you really do need a tactile input system for calculators as they help avoid costly and confusing mistakes.

I'm not sure why there is so much hate for TI to be honest; they are very functional, very robust, have very good battery life and good resale value. I myself however use a $20 Casio 115/991ES PLUS.