What is the most affordable way to lift breasts? Are there any alternatives to surgery?

Thanks for your question! If is difficult to answer without seeing photos/examining you. Speaking in broad terms, a scarless breast lift is best for women with smaller breasts (A and B cups). Radiofrequency skin tightening is a great way to tighten the skin of the breast, the "breast envelope." Make sure you consult with a board certified plastic surgeon and discuss:
-Bodytite with Morpheus to the breasts
-ThermiTight to the breasts

However, patients who have had massive weight loss like yourself often need staged procedures. Skin tightening first, then skin removal and repositioning of the breast tissue to lift. Good luck!

Currently,
there really aren’t any reliable nonsurgical ways to lift and reshape your
natural breast tissue except for a surgical breast lift, or mastopexy. The
appearance of sagging, deflated breasts after pregnancy is a common concern
among women, and is the result of physical changes in the supportive structures
of the breast as well as changes within the breast tissue itself. A breast lift
tightens and corrects these issues so your breast is properly supported again
and is restored back to a front and center placement. There’s simply no way to
do this other than with physical access to the breast itself, and that means taking
a surgical approach.

You probably need a lift and there are really no options other than surgery.  A lift alone will leave you smaller, but with a better shape.  Adding an implant may improve the fullness in the top, but also increases the cost (and risks).  Go see some plastic surgeons and get their opinions.  Good luck!