The 5-car trains would require two transformers to cope with the electrical load that would be placed upon them in powering the traction motors across five carriages.
The fix to cut costs there is to move vehicles between sets, reducing the number of additional pantograph/transformer vehicles required. Rather than needing 2 for every 221, you could do one of the following:
2x 5-car units => 1x 4-car and 1x 6-car. Add three new vehicles to result in a 5-car and an 8-car unit. Average 1.5 new cars per trainset.
3x 5-car units => 2x 4-car and 1x 7-car. Add four new vehicles to result in 2x 5-car and a 9-car unit. Average 1.33 new cars per trainset.
4x 5-car units => 3x 4-car and 1x 8-car. Add five new vehicles to result in 3x 5-car and a 10-car unit. Average 1.25 new cars per trainset.
This assumes that you could run up to 10 cars with two transformer vehicles, given you can run 5 cars with one. It also assumes that moving vehicles between 221 sets is reasonably straightforward.
Of course, the tradeoff is that fewer new cars means fewer new seats, but there are certainly options there.