There seems to be a few services to purchase bitcoins from out there, as in bitinstant, MtGox, bitcoin.de, etc.
I would like to know if someone has looked around to find the one with the cheapest transaction fees for buying/selling bitcoins.
In the U.S., the least expensive method is a Dwolla transfer to an exchange that accepts Dwolla (Mt. Gox, and Camp BX). Dwolla charges $0.25 per transaction when the amount transferred is over $10.
Quite simply, depositing cash at a bank or 7-11, Walmart, CVS is going to be the fastest and easiest way.
One method that was the least expensive, depositing cash at an exchange's bank, is currently unavailable as the exchange that offered that (BitFloor) has suspended that method, temporarily apparently.
BitMe offers this but they don't have much liquidity yet (on the sell side) so that might not work.
If you are looking to deposit $500 or more, then Bitcoins Direct will accept that cash:
All other cash deposit options:
If you are in the U.S., there is one vendor that apparently is offering to sell via credit card.
The fees aren't trivial but you can use a credit card to purchase by using VirWoX where you can buy SLL using your credit card, then trade SLL for BTC:
There possibly are other options that will work for you as well:
Last of all there is #bitcoin-otc where you can connect with sellers via IRC:
For SEPA transfers, Bitstamp may be what you want. I've been using it for months now. There is no deposit fee (just what you pay to your bank for the SEPA transfer), and you pay up to 0.5 % fee when you buy BTC (this was actually free last week but you missed it). There is also some currency conversion expense (your EUR will be converted to USD), but they use ECB's currency rates which are quite good, and don't take any fees themselves for conversion. Also the exchange is quite liquid (third largest USD market) so the spread won't cost you much either.
If you're just buying BTC, this is it. If you plan to withdraw EUR from the exchange, you'll also pay 0.90 EUR per withdrawal.
Also all SEPA transfers are processed in 24 hours, sometimes the same day.