one suggestion i can offer is to price the filter at boats.net, they advertise their pricing is 20% discounted from retail. Then compare to the pricing at partspak.com - all you will need there is the part number from boats.net (partspak's web is way more difficult to look up parts than boats.net)
Partspak's prices are usually (not always) 15-25% cheaper than boats.net but they're shipping prices are outrageous. But what matters is, save the url to partspak's page showing the price, buy it from boats.net and after the purchase email them the price over at partspak - they'll match it by refunding, but will only do it thru a refund process (that sometimes take 2-3 weeks) - they will not do it in advance
2nd suggestion - i've fabricated my own filters using a K&N as the core - i'm assuming the 4518 filter is similiar to the 4514's filter - if you purchase a K&N large enough to cut out the filter area you'll need for the 4518's filter, cut the old paper filter out of a 4518 oem filter that presumably is beyond it's usefulness, and simply cut/fit the filter material from a K&N into the 4514's plastic frame. Be sure to dremel the 4514's plastic frame where the filter will bond to it, degrease thoroughly (MEK, acetone or alcohol) and using one of the epoxys sold in a cartridge or mini-caulk gun style applicator at the Trak Autos, Auto Zone etc, simply lay down a bead of epoxy where the filter meets the plastic frame.
with the K&N filter core, you'll be able to clean and re-use it a number of times
if you think honda's filter prices are high (i agree they are) take a look at john deere's and others - they're all pushing profits to the limits as revenue volume is low
3rd suggestion - i'm using a 4514 filter housing on my 3813, and i didn't like the price of the 4514 filter either - but took another route until i get around to doing the K&N conversion when it's run it's useful life - i made a foam sock from 1/2" thick dense soft foam, and slip it over the end of the housing inlet so it collects 70% or more of the dust before it gets into the housing. I apply a little oil to it before installing it, and each week after cutting front & back yards, i wash it, dry it and re-oil it before refitting it. it's actually given the existing 4514 honda filter a longer life - this is the 3rd year that it's been on there now. End of this season will probably see the conversion to the k&N core
fwiw
