So you've built the $50 robot board - so how do you add the ability for speech?
First the downside - the program is just 'too big' to fit inside an ATMega8
Then the upside - you can replace the ATMega8 in your $50 Robot with an ATMega168. They are plug in compatible - ie you can plug in the ATMega168 instead of the ATMega8 without making any other changes. The difference is that the ATMega168 can hold 16k of program (Speech needs about 12k) whereas the ATMega8 can only hold 8k of program.
Since the program still eats up most of an ATMega168 then I would recommend building a dedicated board. This can use the standard $50 design (with the ATMega168). However: you can save on all those header pins as the only ones you need are Port D0 and D1 (which are the uart receive and transmit). These are the first two header pins at the top left of your microcontroller. You will then need either 1 or 4 I/O pins depending on the mode in which it is driven.The easiest, and cheapest solution, is to use my 1 wire digital mode. This means you could use any one of the other pin headers.