Simplest, cheapest, and totally syn free is a tomato sauce - just onion, tinned tomatoes and your choice of herbs and spices.
And you could bulk it out with any other veg you can think of. Roasted veg? (Peppers, carrots, sweet potatoes, butternut squash, leeks....)
Anything.
Totally free pasta sauces are one of the easiest things in the world!