For prices and counts, 两万 (liǎngwàn) is more colloquial than 二万 — Mandarin speakers normally use 两 before 万/千/百 (两万, 两千, 两百). 二万 is correct but sounds slightly formal or list-style. The exception is mahjong: the tile is read 二万 (èrwàn), never 两万.
culture
In mahjong (麻将), 万 is one of the three suits — the "character" or "ten thousand" suit, numbered 一万 through 九万. 二万 is the 2 of that suit.
