Mine wasn't exactly cheap but it still works nicely after 4 year. I got it from here. 75Ah AGM gel battery deep cycle [SSB]. $240.00.
I run Laptop [19V]+ camera [12V] + mount [16V] + heaters [12V] all night and more.
I bought a 200AH AGM from a mob at Fairfield (Sydney) off Ebay. Their price was very reasonable and delivery was $30, which for a battery that weighed 45 odd kilo's was well worth it.