Seahorses are extremely cool! I am slowly converting my personal 29gallon reef into a seahorse-reef. I have (2) Kuda seahorses now.
If you haven't already, be sure to do a lot of research on proper seahorse care...they have different requirements than your average saltwater aquarium. There are also a lot of different species of seahorses, and different qualities of seahorses...wild caught, tank-raised, tank-bred, etc. Ponies can range in price from $20 to $500, sometimes paying more is worth it, sometimes not.
On my twice-monthly visits to the wholesalers I commonly see inexpensive, 2" $20-$40 tank-raised Keloggi and Kuda Seahorses in black, yellow, and black/yellow mottled. I also (less frequently) see 4" ones in the $60 range, and occassionally see gorgeous wild-caught brigth Orange or Yellow Ponies in the $160 range.