"... we don't have to waste 100 bucks for a clock."
I have seen digital clocks on E-Bay for £10.00 which is about $12.00.
I think a mobile might be a bit too finicky to use in a time scramble.
Hang about a tournament and wait till you hear someone who has just lost say they
are given up chess. (a very common occurrence) Offer to buy their clock for $5.00.