A lash serum and a set of extensions both end with longer-looking lashes, but they are not in the same financial league. Over a year, a serum costs a small fraction of what extensions do - the trade-off is patience versus instant length.
The yearly numbers
A growth serum like GrandeLASH-MD ($36) or Babe Lash ($50) lasts roughly three months a bottle, so a year of consistent use lands around $150. A hybrid set of extensions with 3-week fills runs about $1,900–$3,500 once you add tips and aftercare. That is a difference of well over $1,500 a year.
What you actually get
Serums grow and strengthen your own lashes over 4–6 weeks - a natural, lower-maintenance result you keep even on a no-makeup day. Extensions add dramatic length and curl instantly, but they shed with your natural lashes and need a salon visit every two to three weeks to stay full.
Which is cheaper over a year?
Serums win on cost by a wide margin. If your goal is the lowest yearly spend with a natural look, a serum is the clear value pick; if you want the most dramatic result and can budget for it, extensions deliver that the day you walk out. Many people use a serum between sets to keep their natural lashes healthy and stretch out the time between salon visits.