I purchase them for Stella at www.healthypets.com because they have the best prices I've found. They also sell them in bulk, which allows me to save a little additional amount of money. They come to approx $10 to $12 depending on the size or if you buy in bulk.
The collar is supposed to last up to 90 days. I find this to be an accurate time frame. The collar is NOT supposed to get wet. If it does get wet, you probably will need to replace the collar. A wet collar will loose its effectiveness.
I have found this collar to be well worth it. Having a dog with a full coat means I need to be extra careful watching out for ticks on her. Some peple do not like the idea of the chemicals associated with a tick collar. I however, find that I need to have a Preventic Collar on Stella at all times while she is outside. The ONE time she didn't have a collar on, right after she had a bath- she came in with a tick that just started attaching to the inside of her ear.
The collar is very easy to put on and take off. It has a simple attachment mechanism that works effectively. (See the pic of the attachment part of the collar below.)
This product is very overall wonderful and provides 90 days of tick prevention. My only reservations would be for multi dog households where the dogs like to chew eachother's collars. There could potentially be a problem if a dog ingested the collar.