I have found a few ways to save money on starting seeds. Although seedling trays aren't very expensive, you can sow a lot more seeds using these methods.
Mesh Seedling Bags - I spent less than $8 For 100 bags. These bags are biodegradable, so if you choose to, you can plant them directly into the soil.
Soda Bottles - Dwight drinks A LOT of Mountain Dew. So I save the bottles, cut off the tops and put holes in the bottom for drainage. So if you or someone in your home drinks soda, save the bottles! They're great for things that grow tall like butternut squash, cucumbers, etc.
Apple Sauce/Fruit Cup Containers - Perfect size for seed starting. Just remember to poke holes in the bottom.
Baby wipe containers & Press n Seal Wrap - Not only do I use wipes on myself, I have a toddler and a newborn. So I have wipe boxes everywhere. I break off the tops and put the mesh seedling bags and soil pellets inside.
Then, I cover them with Press n Seal to make a little green house. It helps keep the heat in for better seed germination.
So...no fancy grow lights, no expensive heating mats or ither equipment. I simply used everyday items I have at home.