Consider custom post type of ‘Contest transactions’ parent posts could be the contest, then you can use the post meta
meta_key => initial signup, meta-value => +5
meta_key => refunded_order, meta-value => -12
and then you could a query adding up all the transactions.
I prefer to use wp tables and structures where possible as then many features, functions and other plugins are available to use to work with and report on your data. If you go with custom tables, you have to write everything yourself.