I typically buy a load of high-quality Red River topsoil from our nursery every spring. It is about $50 for a Bobcat scoop full, which fills up my 4X8 utility trailer. Of course, that is cheaper per cubic foot than buying potting soil in a bag.
Having said that, I thought the best method to start seeds and plants inside the house was to use a light, fluffy, potting soil or seed starting mix in starter pots.
I tried to use topsoil a few years ago for starting plants inside the house and it did not work out very well for me. The topsoil had too much life in it, and I got bugs on the plants. The plants did not grow all that well in the pots which I think was due to the topsoil being too heavy. I also had problems with mold, but I think that was because I overwatered the pots.
Anyways, I switched over to using a potting/seed starting mix in my starter pots. I use the 3-inch net cups and bottom water my pots. The potting soil mix wicks up the water and I don't end up overwatering my starter plants anymore. The added growth fertilizer or ingredients used in the potting soil has resulted in plants 2X-3X taller and stronger than the plants I started in topsoil only. Since then, I have not even considered going back to topsoil for starter pots.
I am open to better options, especially if I can save money. I grow my tomato and pepper plants for 8 weeks inside my house. I don't have an outside greenhouse. Do you use regular topsoil for your seed starting in pots inside the house? Are you successful with that method?

Maybe I just did it all wrong with the topsoil when I used it. But I have had great success with using high quality potting soil mixes. I know a small bag of potting soil is very expensive compared to a load of topsoil, but if the starter plants don't do well in the pots inside the house, they don't do as well in the garden outside when I transplant them.

When it comes to gardening, I have probably had more failures than successes. I am always looking for better ways to start my plants, which I think is probably the most important thing for better harvests at the end of summer.