Run Wikia

Unlocked after beating Level 3, the shop is where you can go to buy characters (like the Skater for example), upgrades, and costumes. Currently, the shop has 17 items:

Item Cost Notes
Respawn costs are reduced by 10. 1,500 The first respawn becomes free!
Unlock the level editor. 500 Currently only available on PC flash versions.
Unlock the Skater. 300 Also unlocked by beating Level 10.
Unlock the Lizard. 600 Also unlocked by beating Level 40.
Unlock the Child. 2,000 Also unlocked by beating the Low-Power Tunnel.
Unlock the Student. 10,000 You never do see what's in the backpack directly.
Unlock the Angel. 12,000 He is indeed a dashing fellow, not appearance wise though, he actually dashes in the air.
Unlock the Duplicator. 6,000 Costs 25,000 on mobile
Unlock the Pastafarian. 6,000 Costs 25,000 on mobile
Unlock the Bunny. 2,000 Also unlocked by earning 8 achievements.
Unlock the Gentleman. 2,000 Usually a good early purchase due to his ability to fly towards power cells.
Find more boxes in Infinite mode. 200*2n

(n= amount of times the upgrade has been bought)

There is no maximum amount that can be bought; it doubles in cost after each purchase. Also, curious how this helps you? Well, boxes cover holes, so it can reduce your chances of falling. Also, if you use the Duplicator, you can produce duplicates, or if you use the Pastafarian, you get more space on the light bridge.  Each one makes the boxes spawn slightly more frequently, but not by much. More importantly, the boxes continue to appear further in Infinite Mode, about 200m further per upgrade. The later upgrades to this are generally not worth it, unless of course every other item in the shop has already been purchased, and the player has amassed more power cells than could reasonably be spent on respawns making power cells almost worthless.

Boxes in Infinite mode will sometimes contain power cells. 2,000 This means that if you get close to a box, you may get some power cells.
Unlock the winter costumes. 500 Also unlocked by completing the Winter Games.
Unlock the Halloween costumes. 750 Also unlocked by completing the Low-Power Tunnel.
Unlock the Pirate costume. 1,000 Only appears in the shop when you have bought the Pastafarian. Does not count as a winter or Halloween costume.
Balance on razor-thin edges of tiles. 1,000 An experimental upgrade in the HTML5 version.

The specifics are currently unknown, but the following characters can flip gravity on the edges of tiles.

The Runner and her costumes, the Skater and his costume, the Bunny, and the Angel.

Unlock permission to refer to the Power Cells as batteries. 15 Unlocks the cutscene Batteries. Clicking on this in the shop when you have bought it shows the cutscene again.

This is what the shop page looks like when you first unlock the shop:

Shop Updated Images (Incomplete)

And after you unlock everything (except for the "find more boxes" item):

Shop Updated Images (Complete)