Button VoltsB1 1.0B2 1.5B3 2.0B4 2.5B5 3.0B6 3.5B7 4.0B8 4.5
The R-2R ladder should work. Put a buffer between the switches (pulled to Vcc) and ladder inputs, then analogRead the ladder output. The 74hc244 logic IC has exactly 8 buffers and costs around $0.50.
Using of some dedicated I/O extender is out of question, as it is expensive and as well require more than 1 available pin to connect.