You could take a train from Kanchanaburi to Nam Tok from where you will be able to get a songtaow/taxi most of the way. However, the hotel will have to transfer you across the river. You could also do it by 8203 bus. However, you will need to get from the main road from where the bus drops you down to the pier.
Not stayed there but I would think the resort will transport you from the pier to the main road, from there the cheapest way is bus, takes about 2hrs and think it's 50bt
The bus is also the most convenient way since the service is much more frequent than that of the train.
The bus is the cheapest and easiest.
Thank you all for your ideas and information : )