On Siren, the cheapest way to get guild points off these is to buy them from the auction house, no doubt about it. Someone else will likely snatch them up before you though, so you may be forced to make your own from the pricy materials. However, you can help to lower the price of the harps by desything your own coeurl whiskers.
For reference, I am 82 woodworking. I grabbed a stack of lightning crystals and headed down to bastok markets music shop. My desynth was lightning crystal+Maple harp(Cap about lv 23). The harps are only sold there if bastok is in second or first place, it appears. Maple harps cost about 40g each. Out of the 12 crystals, I got 3 maple lumber and 4 coeurl whiskers.
It is known that you can get 2 coeurl whiskers, a coeurl whisker, or a maple lumber from this desynth. The information on the various sites I have looked is mixed, and I don't know the difference between a lightning crystal hq and nq, so I am unable to determine actually which are the hqs. You can also use the desynth of longbow+lightning crystal for coeurl whisker, but it I wouldn't recommend that. You only get coeurl whisker from it for HQ2. There is also a HQ3 that is not the whisker.
I bought my ebony lumber and logs from the auction house, since unfortunately the guild was too pricy at the moment. It would have been cheaper had the guild price been lower at that time(If you didn't already know, the amount purchased on the previous game days increases the current price). In the end, I had paid about 120k for 6080 guild points. Not TOO bad.
Man if you read this post that is soo cool cause like i like to play this harp. It is very soothing man. I mean like i can listen to this harp and it soothing and like i can waste people with this man. If only i could give this away and someone else could listen to it man but i have only seen one so i am not going to do that man.