A far easier solution to the problem is to list the item without a reserve, starting the auction off at a price that would ensure that you got the minimum amount that you were happy to accept once eBay and PayPal have taken their cut of the selling price. That way if somebody places a bid you can guarantee that you'll definitely get the very minimum price that you'd be happy with, or more if you've got two or more bidders regularly placing bids for the item and driving up the final selling price.
With regards to whether or not eBay charge extra fees for increasing or lowering the reserve price I couldn't say for certain whether they would as it's not a feature I have ever used on any of my listings, although there are some things that eBay charge you for and other things that you won't incur any extra fees for. As an example of this, eBay would charge you a bit more if you wanted to include Gallery Plus on your listings so that potential buyers could get a really good zoomed-in look at the shots so as to get a decent close-up and inspect the item thoroughly, whereas if you were just doing something such as adding an extra postage option, such as Royal Mail Guaranteed Next Day Delivery, then eBay wouldn't charge you any more for doing that.
At the end of the day the best option is always to start your auctions at the lowest possible price that you would be happy to accept once eBay and PayPal have taken their cut of the selling price. That way you can ensure that your reserve price has already been met the moment somebody places the first bid. If the item doesn't sell then you don't pay any selling fees, whereas with the reserve price option you have to pay the fees for using the reserve option on the listing, even if the item doesn't sell. To me it seems daft to sign up to something which is, in effect, like voluntarily signing up to hand eBay more money from the listing than you need to.
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