Just for electricity the price difference between a 150 and a 250 will be about $7 each per month. So let's call it $15 per month for the pair. Depends on electricity prices, obviously.
Electricity costs per month for...
2X150de=$20
2X250de=$33.50
2X400se=$53.60
Costs for bulbs(about, don't have exact prices) Replace every 9 months
2X 150de= $300=$33.33/mo
2X250de=$320=$35.55/mo
2X400se=$180=$20/mo
So, total
2X150=$53.55/mo
2X250=$69.05/mo
2X400=$73.60/mo
Cost/watt/month(Dunno how important this is given the differing efficiencies of DE and SE)
2X 150de= $0.1785
2X250de=$0.1381
2X400se=$0.092
I've seen info that says that 250 watt DE is up near the same intensity as 400 SEs, so I'd call 250 and 400 pretty much equal. Maybe still a slight advantage to the 400s, but not enough to make rule out personal preference as a deciding factor.
Now, I've started seeing ads recently for 400W DEs, those would be massive for light.