A problem has appeared in a group I run. Almost every five minutes a player interrupts whatever's going on to ask a question that has NOTHING to do with the situation they are supposed to be paying attention to.

Example: players were on a boat fighting a muddled purple worm (basically a purple worm but underwater instead of underground).
Player 1's turn: "oh, hey DM, how much would it cost for a harpoon gun?"
Me: "uh, I'll look it up later, but you're in the middle of a fight and can't buy anything."
Next player's turn: "Hey, DM, those enchanted floating axes I wanted, how much did you say they would cost again?"

This happened almost every other turn in the combat. A fight that was supposed to last for an hour, maybe an hour and a half lasted the whole three-hour session. Any advice for what to do?

I'm just spitballing, I don't know you guys, but it sounds like you might be that rare DM who is more interested in fights and tactics than their players are.  The fact that they'e interrupting you with game-related questions is a high-class problem, imo.  You need to worry when they're just quietly looking at screens.  But the questions they're asking suggest that they'd rather do something else (shopping, downtime crafting, RPG, whatever) besides having a 1 1/2 hour fight scene to begin with.  

You see the mirror image here all the time.  DMs who have these ornate intricate personal stories to tell but they're frustrated that the paladin has a walking-around AC of 27 and isn't stopping to sightsee.

So, just based on that, my advice would be to simplify combat scenes as much as you can and still respect yourself in the morning; and, if that isn't enough?  To find another group.

That’s definitely an odd problem to have.  When the DM tells me to roll initiative I’m definitely not thinking about buying harpoon guns anymore.  

I take it you have tried talking to them about it, like “Hey guys, could we please stay focused on combat?” If not start there.  

If that doesn’t solve your problem, well then I’m really not sure what would.  Maybe try promising “we will get to that, let’s finish this fight first...” (hint hint)...

I would just ask him right there at the table, "why are you asking me about where to buy a harpoon gun at while you are in the middle of the sea?".  "Why are you asking that question now, (whatever off beat question he asks).   "Do you prefer to be doing something else? "  I want you to enjoy the game and can set it up to your GUY'S liking.  Talk about it as a group.  He may rather solve political intrigue, or murder mysteries rather than combat.  

What I've done in that situation has been to tell my players that they don't know that information and will have to find out when they return to town.

"Hey DM, how much would X cost me?"

"I don't know, find someone selling X and ask them."

~

"Hey Dm, how long would Y take me?"

"You haven't done Y before, so you'd have to try it and find out."

~

That is usually enough to get my players to RP more when they are in situations that fit what they are asking, and unless it is something fundamental to their character (Like enchantment time for an artificer) it works just fine. 

There is also the classic, "Now is not an appropriate time to be asking that question."