How many times will Desecrated Tomb's ability trigger when cost for Bone Dragon's ability is paid?

The common sense tells me that it is going to trigger once, since the cards will leave graveyard simultaneously. However, according to rule 601.2h and 602.2b, activation cost can be paid in any order, implying that there is an order in paying cost, therefore cards are exiled from graveyard one by one, making Tomb's ability trigger seven times.

Am I getting something wrong? What are your thoughts?

Desecrated Tomb will trigger once and only once when you activate Bone Dragon's ability.

Activation costs are indeed paid in any order, but the two components of the cost of Bone Dragon's ability are “{3}{B}{B}” and “Exile seven other cards from your graveyard”, respectively. Those two components are what you can pay in any order–either you exile cards before you pay mana, or you pay mana before you exile cards. You cannot split one of those costs into multiple separate components and pay each component separately one by one–that's what 601.2h is saying when it says that “Partial payments are not allowed.”