Electric wall-switch boxes? The blue-plastic 4-gang is $3/box, tho you may not find a 4-gang blank cover. The RACO steel boxes go $10/box in 4-gang, so not in your budget, and again no covers.
Home dePot lists:
Cantex 4 X 4 X 2 Junction Box Model R5133705 $6.37
Carlon 4 In. x 4 In. x 2 In., Gray, Junction Box Model E989NNJ-CAR $6.37
Cantex 4 X 4 X 4 Junction Box Model R5133709 $8.74
Carlon 4 In. x 4 In. x 4 In., Gray, Junction Box Model E987NR $8.74
Cantex 6 X 6 X 4 Junction Box Model R5133710 $11.07
Carlon 6 In. x 6 In. x 4 In., Gray, Junction Box Model E987RR $11.07
Raco 4 In. Square Box, 1-1/2 In. Deep, 1/2 In.& 3/4 In. Side Knockouts, Welded Model 8189 $0.97
Raco 4 In. Square Box, 1-1/2 In. Deep, 1/2 In. Side Knockouts, Drawn Model 8190 $1.19
Carlon Two Gang, Blue, Blank Electrical Nonmetallic Tubing Box Cover Model A400R-CAR $1.12
Raco 4 In. Steel Square Box Cover, Blank Model 8752 $0.57
Carlon Double Gang, Gray, Nonmetallic Weatherproof FS Box Blank Cover Model E9802CN-CAR $2.47