Are there any more affordable alternatives to ICL for a 21 year old with thin corneas?

ICL is clearly your best option
to fully correct your vision as safely as possible. But, as you correctly note, the cost is higher. Your options would include a surface
laser ablation such as PRK, but your surgeon would need to limit the amount of
corneal tissue removed to minimize the postoperative risk of ectasia
(de-stabilization of the cornea).
The amount could be limited by either intentionally not fully correcting
your myopia (ie leaving you slightly nearsighted, perhaps -1D to -2D), or by
limiting the diameter of the optical zone (which reduces the ablation depth,
but increases the risk of night glare and halos). My recommendation would be either to wait until you have the
funds for ICL, or see if your surgeon offers financing where the ICL procedure
can be paid off over time.

ICL surgery is the best option in this scenario. Many practices offer financing options to make the procedure much more affordable. The investment in excellent vision would be well worth it for you.