What kind of lentils?

Probably a good thing to get you started on them is to add them to a bolognese sauce. So use 250gm of mince, 1 can of lentils (no need to soak, costs about $1, easy peasy) and 800gm of shredded mushrooms and cook your bolognese from that.
You will cut down your meat, cut down your fat...and cut down your calories. Good all round. And cheap as chips.