How many resistors do I need for an 8 switch DIP switch block?
SMD resistors are as cheap as dirt. Especially if you would buy full roll of let's say 10k ones (i.e. something widely-used, so that you could use in multiple projects). It's hard to overload pull-up resistors, as they can dissipate 0.125W usually. 

There are also 4 and 8 resistors in 1 package, this will be even cheaper if you are on large scale.

You cannot use single resistor for multiple wires - as they will be shorted (or you will need diodes :-D )