How big do you want the P-51 what model ? the basic airframe if its Balsa and ply can be done within reason the costs add up when you add some things that are a little hard to make . If your doing a P-51 D the canopy will cost you a bit . If your not good with fiberglass the cowl is another cost. Then the retracts , Almost the price of a good used engine. Then servos and radio gear.
Before I start a big project I sit down and do a complete cost breakdown. This way there are few surprises when I commit my time and hobby dollars.
The more you do yourself including hunting for deals on what is need can save you money . This takes time Its still going to cost you. Spread out over a reasonable building period you may spend a bit more then expected but its doable.