Your mom

The National Parks.

I guarantee you, for the amount you pay in your taxes for the upkeep of YOUR National Parks, you get orders of magnitude more beautiful land to enjoy than the acreage you could possibly purchase directly.

Before Trump fucked it all up that is, of course...

Outside of the awesome 'national parks' answer from someone else, I would have to assume the best cost-to-surface-area purchase in the world would be really cheap land in the American West, Australian outback, Russian tundra, Canadian North, etc. Assuming it doesn't have oil on it, some of those areas, land practically given away. Sometimes you can get governments to pay you to take it on and try to do something useful with it.

If you consider that ownership usually includes mineral rights for miles under the ground, this really starts to look like the obvious choice of your looking for volume, not just area.

You shouldn't think big, necessarily, you should think heavy. Postage to be paid on delivery.

Lead sheet in a return envelope is a great thank you for unsolicited application offers.

Heavy shit sent to UKIP is a classic one.

You can order packing peanuts for ~$5/ft^3 and some places will do free shipping over a certain value. There are also decorative balloon bunches, though I don't know about price there.

Liquids would also work. Heavy, sloshy, awkward, etc. and you don't want to drop them because it'd make a mess.

Hay bale - or multiple.

How much do you want to spend? Used shipping container?

Drywall. Heavy, fragile, 2 out of 3 dimensions are relatively large. Easy to transport when you're stacking them in a truck but difficult up the stairs for example.

Bubble wrap. Cheap-ish per m³ if taking up space is more important but Easy to carry a roll of that around alone.

Horse manure tends to be free around these parts

By area activated charcoal has a ton of surface area

I am curious what the purpose of this is

Air, cuz it's free, and by size the biggest object in our planet

Heaps and heaps of sand?

Whatever you end up doing, OP, you owe us a pic or video of the result!

A house in an Italian village for €1.

I know this one, it’s skin!

A big fuck-off mylar balloon.

The $18.6 million in cash and 225 gold bars in two boxes that belonged to the late Mr. Jimmy Lemi Milla.

A really big sheet of paper?

Does it have to be usable?

I think if you're looking for largest volume of material to dump on your enemy per its cost, your best options are either going to be dirt or possibly a huge amount of expanded polystyrene, i.e. styrofoam. If you're doing this outside I think dirt is a better option, unless you can find somewhere to source really huge blocks of styrofoam. A thousand cubic feet of packing peanuts (approximately $2714) would certainly be funny, but they'd probably be prone to blowing away. That's no good if you want the mark having a tough time getting rid of them.

What budget are we working with, like what's the most you'd be happy to spend on this?

Bouncy castle

By area, my first thought was something like gravel. Relatively cheap, and covers a large area. However, delivery is relatively simple. They just load it up to a dump truck, and they dump it where ever. For things that are pain in the ass to deliver, I think about wind turbine blades and such, which aren't exactly cheap. I think if your goal is to spite the delivery people on the cheap, I'd say cat litter. They don't come in bulk the way gravel does, so someone has to move it box by box.

Eat fiber supplements for a day. The delivery won't disappoint.

Can you still buy a star? Obviously dubious that you actually own it. But certainly bigger than anything on earth and a bit tricky to deliver.

Bedbug ridden camper with no axles. A tow company with a flatdeck could easily load and unload still, but if you got it delivered while they were not home it would be a nightmare. Bonus points if it's an apartment complex and you can put it in their stall.