I would love to have this

What kind of price would DMs put on this or what kind of efforts to make one?

I think if a character has weaver's tools and another has arcana they could attempt to create it using the rules for creating magic items in Xanathar's Guide to Everything.

Quote from coachmanak >>

What kind of price would DMs put on this or what kind of efforts to make one?

 40000 gp to buy 20000 gp to make, source: DMG and XGtE buying and making magic items.

I think it should cost 20,000 gp

In all the reading I've done item creation takes half the cost but u have to be the proper class or be able to cast the spell. In my games if they want to create an item it cost half the price but the remainder of the the normal price is how many days it takes to create. So if the carpet costs 20000 gold to make then they have to have 10000 gold and it takes them 10 days to make (1 day for every 1000 gold). If they can't cast the spell but have a high enough lvl the same way the material is already in the scroll so they just have to put the time in.

Could I fit a 6 ft. x 9 ft rolled up in a bag of holding?

The bag of holding extradimensional is described as a four feet cube. The longest distance from opposite corners is the square root of (four feet squared times three)
or 6.9 feet

Does it require an action to command?

It says, "You can speak the carpet's command word as an action to make the carpet hover and fly." So yes you have to use an action to command it. 

I CAN SHOW YOU THE WORLD!

I understand it requires an action to activate it, but does it require an action to move it afterwards?

Yup. Any time you want it to fly, including on subsequent turns, requires your verbal command as an action. 

However, since it doesn't say that it can only move one turn at a time, you could say "go east" and it'll keep moving up to its movement speed each turn in that direction as long as you are within range. 

Kinda makes travel a bit insane. You could be asleep while riding it, so you basically would never need to stop for anything other than bathroom breaks unless you have a VERY unlucky bag of holding. In that case, you could just keep traveling without needing to worry about being ambushed by bandits in the middle of the night. 

Still need to worry about flying monsters though. Damn harpies. 

This is a really cool item.

According to Xanathar's Guide to Everything, a Very Rare magic item would cost 20,000 gp and take 25 weeks of work. I would probably have a few Arcana + Weaver checks to determine the end size. Probably tie some item (like an elemental mote from the plane of air) to the recipe so the party has to go on a quest for materials

A possibly more cost-effective way to get a magic carpet would be just using creation bard’s animating performance on a rug.

My understanding is an action to start it going, once its flying speaking is a free action and you can fly around as you desire using verbal commands. 

If a creature is on the carpet of flying,  and was hit with something like a Warlock's repelling blast, would that creature be knocked off the carpet or would the carpet get knocked back with the creature?

Is the command word a secret? Does it vary by carpet? Is it written on a tag along with washing and care instructions?

What about making it sentient, a little immature, playful, perhaps it's not so easy to ride and takes practice or the occasional dex check to stay on?