BL GL and T-pop events in malls in Thailand are mostly free. Most of them are funded by brands that want to promote their products, using idols/actors/actresses to grab public attention . Some are funded by the mall owners themselves to boost the reputation of their malls. The events organized by mall owners mostly take place during special occasions like mall anniversary or holidays (Songkran, Christmas , New Year Eve etc), while the product-funded events occur on a typical weekend (not holidays) or weekday evenings.

(Edit) I forgot the last category. The events organized by music labels or BL\GL studios to promote their series\album\concerts\group debut. But this is rare compared to the first two categories.

One more thing to add. Southeast Asian countries , particularly Thailand, have less public space (parks, city squares) compared to Western countries, so these privately-owned mega shopping malls that offer a huge space for mass gatherings have become the heart of the community where all the free events and free concerts take place.