Last Respects' Base Power would be 300, but its Damage would be around 600 Power.
Adaptability increases the effectiveness of STAB moves from the usual 1.5× to 2×.
Last Respects increases by 50 BP for every Pokemon fainted, so after 5 Pokemon faint, its Base Power would be 300. Adaptability is a damage modifier that provides a 2x STAB boost (300 x 2 = 600), doubling the damage, which does almost the same thing as doubling the Base Power.