Perhaps, if you are a bit short of money, or have other things that you might need before this, you may consider not buying the biretta. I don’t think the biretta is worn as day-to-day wear (I think the one worn like that is the capello) for diocesan seminarians, but only during really solemn or choir functions. As a seminarian, there wouldn’t be all that many chances you would have, therefore, to wear it. You can’t wear it, for example, when you are serving Mass. Even on the chances you have, they may not let you since your fellow seminarians may not have birettas and you would stand out.
Perhaps you might think of delaying such a purchase until you get ordained to the diaconate or the priesthood. A priest has more opportunities during the administration of the sacraments to wear it.