It's released for £6.99 or your regional equivalent. Right now it's got 20% off for the launch sale so it's £5.60 and AoE2 HD edition is £3 for this weekend only. All on steam btw. And yeah, HD edition is worth getting even if you own regular. It adds more to the factions, new techs, bugfixes and all the usual pretty things from a HD update.
EDIT: Oops,new factions are Portugal, Berbers, Malians and Ethiopians. Each come with a fully voiced campaign and there are some new generic units and techs: Arrowslits is a tech that increases tower damage, Arson improves melee units damage vs buildings, fire galley is a feudal age fireship, Siege Tower hides units in and helps them scale walls...Did not know that scaling walls was a feature, might have to get it just to see that.