Ding Ding Ding! Oh I love that sound! The answer is indeed... Fire Ring.

This doesn't cost a lot of money as it's free
Obviously it's a free item and the location (Costa) is written in this line

Down underneath where the sunshine can't reach
This hints that the location has a lot of sunshine and the item is in a basement.

In a roundabout way, it will keep you warm
The item is round and this hints it is fire-based.

A super-hot curry brings your bum hole to harm
Having a pooh after a hot curry can be painful. This is affectionately known the UK as a 'Ring of Fire'.

Jinxie baby, the floor is all yours.

EDIT: Sorry Cleric, Jinx just got in there before you!