Hi tefields99,
How much money to bring is very much up to the individual but we would recommend planning to spend about the same on things here in Costa Rica as you would in the US. Whatever you decide to bring in cash, yes, we would definitely suggest bills of $20 or less. Cashiers checks are not widely accepted here but you can use your debit card at ATM’s as long as you have previously let your bank know. Here is a post we did on the topic of money that might come in handy: Money Matters: Currency, Exchanging Money, and Tipping in Costa Rica. You might also like: Cost of Traveling in Costa Rica.