Personally, I would go for something that is good for you, and not 'pricey' for them.

Weekends are their money income times, so don't be greedy. Suggest a once-a-month 3 day mid-week stay, for the next 6 months. Any pics that you take during those visits can be used ( non-exclusive ) by them, free of charge, with you credited as the photographer. Chances are that they are not fully booked mid-weeks, so they are not losing anything having you there, you get a monthly break, you're an online consultant so you should be able to work online sitting next to the dam, etc, etc.