Depends on your rules.
Hard to beat the value of an Ollie with a 310 in it for HP/$ setups. IF it's allowed.
Or an IH 560 with RD450/501
Or a MM with HD800 power unit.
If you can't do transplants, you can take most engines and double or triple the HP for $2000-6000. Beyond that it starts getting more $/HP.