Length (l1) of bigger box = 25 cm

Breadth (b1) of bigger box = 20 cm

Height (h1) of bigger box = 5 cm

Total surface area of bigger box = 2(lb + lh + bh)

= [2(25 × 20 + 25 × 5 + 20 × 5)] cm^{2}

= [2(500 + 125 + 100)] cm^{2}

= 1450 cm^{2}

Extra area required for overlapping

= (1450 x 5/100) cm^{2} = 72.5 cm^{2}

While considering all overlaps, total surface area of 1 bigger box = (1450 + 72.5) cm^{2} =1522.5 cm^{2}

Area of cardboard sheet required for 250 such bigger boxes = (1522.5 × 250) cm^{2} = 380625 cm^{2}

Similarly, total surface area of smaller box = [2(15 ×12 + 15 × 5 + 12 × 5] cm^{2}

= [2(180 + 75 + 60)] cm^{2}

= (2 × 315) cm^{2}

= 630 cm^{2}

Therefore, extra area required for overlapping = (630 x 5/100) cm^{2}

= 31.5 cm^{2}

Total surface area of 1 smaller box while considering all overlaps

= (630 + 31.5) cm^{2} = 661.5 cm^{2}

Area of cardboard sheet required for 250 smaller boxes = (250 × 661.5) cm^{2} = 165375 cm^{2}

Total cardboard sheet required = (380625 + 165375) cm^{2} = 546000 cm^{2}

Cost of 1000 cm^{2} cardboard sheet = Rs 4

Cost of 546000 cm^{2} cardboard sheet

= Rs(546000 x 4/1000) = Rs 2184

Therefore, the cost of cardboard sheet required for 250 such boxes of each kind will be Rs 2184.