I think I have a decent idea of what Meroron's actual price for the lock is now. Meroron's deepest desire might have been something as simple as becoming someone of light and warmth, like Purirun, rather than being cold and dark like her current self. She was probably bothered by the fact that she couldn't go kirakira, even before she met Purirun. She loves Purirun immensely because she dragged Meroron out to the light, but Meroron still felt like a denizen of the darkness and was overly reliant on Purirun to feel the light. She wanted to become someone who could shine on her own. This didn't really come to mind to me before since I assumed her deepest desire would be more Purirun centered due to her obsession of her, but the last episode being about her becoming more independent gave me this idea.
The reason why the lock reacted when it did is that openly admitting that she wanted to be friends with the other Cures would be the first real step towards becoming the Meroron she really wanted to be.