Effectiveness of lifting lighter? Also cheap dumbbell recommendation?


So I'm doing FB Fit and only have one set of 5 lb dumbells, a set of 8 lb dumbbells, and a 6 lb medicine ball. I feel like such a cheater doing "deadlifts" with only 8 pounds per hand when Daniel is doing 50 a hand and Kelli is doing 24... Is it such a terrible thing if I do a couple exercises with a very light weight? Or is it worth investing in a set of adjustable dumbbells? I just can't seem to find any for less than $150... Thank you!