Learn Dutch: Ik denk het hotel is hier even duur als daar. - I think the hotel is just as expensive here as there.

Because we dropped the conjunction dat (that). If we included it, the sentence would be Ik denk dat het hotel hier even duur is als daar, pushing the verb is to the end of the subclause. When you drop dat in spoken Dutch, the second part of the sentence reverts to being an independent main clause with normal Verb-Second (V2) word order.

Can I drop dat and use this V2 word order after any verb, just like dropping "that" in English?

No, dropping dat is mostly limited to informal, spoken Dutch and specifically follows verbs of thought, speech, or belief, such as denken (to think), zeggen (to say), or vinden (to find/opine). For other types of verbs, or in formal written Dutch, you must keep dat and use the standard subordinate clause word order.

I always confuse als and dan in comparisons. Why is it als here?

You use dan for unequal comparisons, such as duurder dan (more expensive than). However, for equal comparisons meaning "just as... as", Dutch uses even ... als or net zo ... als. Since this sentence states the hotel is equally expensive in both places, we must use even duur als.

Could I replace even duur als with net zo duur als in this sentence?