Interestingly enough, I believe I have found the maximum level that can be achieved before WP prices start super-inflation. Erika was able to reach Lv54947 before her price began going up massively for each level. After the WP price of a girl reaches 242Q, they begin increasing by at least 1Q per level, which becomes very expensive very quickly. Before that, it would take almost 100 levels to increase the price by 1Q. Still, almost reaching 55K wasn't bad.