A bag of seeds shouldn't run you more than a dollar. a single starter plant that's maybe 1-1.5 months old should be $2.50 - 3 each. Sometimes there are bundle deals at the nursery like 4 Tomato plants for $4 or something. They're all different prices, and some will be cheaper than my estimate above.
When you say "how much more"...if a bag contains 30 seeds for $1 and a single starter plant is $2 then that means a starter plant is 60x more expensive than a single seed. The fact that you probably only need a few plans per year and can expedite the process by 1-2 months is where the real value is. Someone spent their time raising that plant and watering it daily & using their shelf space in a greenhouse to get it to that point. You get to see the exact plant you're going to get rather than planting seeds and hoping for the best. You'll get those bonus weeks of peak veggie production as well.
If you're concerned about saving a few dollars you can always grow your own starters as I suggested, and that is how the serious horticulturalists go about it. Set up a card table or something near a window and buy maybe 40 Solo cups. Your house/apartment oughtta be warm enough in March to get started on some plants, it's sort of the best of both worlds. Seed prices, but you'll have ~1 month old plants when it comes time to stick them in the ground in April (may be sooner or later than that depending on where you live).
Hope this helps.
