costa rica AUC - how much money should I take

Hi - I went on the Discover Costa Rica trip a couple of years ago and like you was also a solo traveller. The excursions in Monteverde (canopy tour, zip lining) and any extra charges while staying in the hotels/lodges (excursions organised through the hotel, meals, bar drinks, laundry etc) could all be paid for using a debit card. We also had the opportunity to visit the ATM while staying in Manuel Antonio. I found I only spent the money I took on buying water and drinks, any meals not provided by the hotel (approx £20 - $30 a day), a few excursions not organised through the hotels and tipping our guide and driver at the end of the trip (approx $50). You will also need to keep back some money for paying the departure visa at San Jose airport.   

thanks Hannah, which extra excusions would you recommend ?

I would definately recommend going on the trip to watch the turtles nesting while in Tortuguero although this is dependent on the time of year that you are travelling. The canopy tour in Monteverde is also worth doing along with zip lining (if you like this kind a thing - a few of our group tried it and didn't enjoy it). Depending on which excursions you chose you only get the oportunity to do a couple while in the cloud forest in Monteverde (due to the time each one takes). It was also interesting having a tour around one of the coffee plantations in Monteverde.