The fixed price is impossible to achieve, maybe having average price - but it will also change frequently. The reason is about the usage of pokemon in the meta pvp game, the ratity of pokemon and so on. For example, if you have buterfly god, it will never having people buy it, although they can buy cheap for goldencity quest (common and don use it in pvp). Generally, the price is up belong to the usage of pvp of the pokemon.
The factors to desire price of pokemon is always changing, so the price change also. The supply of seller to these pokemon also impact to price.