they have admission cos they are freq by tourist(aka tourist attraction). if ur looking for Wats w/o admission+convenient then Wat Pathum Wanaram fits best.
*if ur open to taking a 60-100baht cab ride, alot more Wats are open to ur choice, especially if your looking for "serenity"
S$1-$4, Wat Phra Khew most ex @S$20 as its bundled with Royal Palace admission. try to go early before the bus loads of tourist flood the place..