Can you use a mana ability in the middle of paying costs for a spell?

When you cast a spell, you must pay its total cost. CR 601.2 lays out the casting process, and CR 601.2h specifies that you pay the costs in any order you choose. Crucially, CR 605.3a states that mana abilities can be activated at any time a player has priority or whenever a player is paying a cost — this is a special exception that bypasses the normal timing rules.

This means you can tap a land (or use any other mana ability) in the middle of paying costs — for example, you can sacrifice a creature as part of an additional cost, then tap lands to pay the mana cost, or vice versa. You are not required to have all the mana ready before you begin paying; you can generate it as you go.

Mana abilities themselves do not use the stack (CR 605.3b), so activating one mid-payment does not create an opportunity for opponents to respond or interfere with your payment. The process of paying costs is not an action opponents can interrupt.

Concrete example: You cast Ashnod's Altar's activated ability to sacrifice a creature for 2 colorless mana, then immediately use that mana (plus a tapped land) to pay the {3} cost of Divination — all within a single cost-payment sequence. This is perfectly legal under CR 601.2h and CR 605.3a.

Note that only mana abilities enjoy this privilege. Other activated abilities cannot be used in the middle of paying costs; they would need to be activated before you begin casting the spell.