Sims Nieuws has discovered that Chinese site My Sims World, is claiming that Hidden Springs will sale for 2000 SimPoints which is $20 US. Please note that they did not cite their source, thus this may or may not be true. But if this is true, that would make Hidden Springs the same price as Barnacle Bay.
Some Spanish sites are saying that it will cost 1.000 SP. I think it’s not coincidence that many pages are giving the same information and possibly it comes directly from EA Spain.
http://www.meristation.com/v3/des_noticia.php?id=cw4e3ab72f3b80f&pic=GEN
http://www.islabit.com/30859/nuevo-dlc-hidden-springs-para-los-sims-3-disponible-el-25-de-agosto.html
http://www.juegosdb.com/los-sims-3-nuevo-contenido-extra-con-hidden-springs-pc/