Generally along with cheapest you are going to find with green price is related to quality not all the time but it is a fairly good guide. Can't think of anyone buying sports cars that bag up and sell small quantities of Green to home roasters on the profit margins.
Best bet to keep the freight down is order a little bigger batches so whatever freight is lower/kg. As someone in a similar situation to you (out of a capital) I do tend to buy 5kg or larger lots normally.
Apart from that if your a student and on a budget it's a bit of a tough gig paying more than you have to but it's worth it for the variation and taste experiences offered.
Pricing on coffee can vary up to 600% depending on the grade and availability (exclusiveness) of the bean. Coffee prices are pretty much set by a number of factors - differential on settled contracts from the C-index, financing, storage, risk, freight, etc.
Green bean supply is by no means a margin or value-based business, it's a scale business. If you think that one suppliers is "more expensive" than another, you will find that almost impossible to prove given that suppliers will *RARELY* stock the same precise bean, e.g. it might be a Ethiopian Yirgachaffe GR2 washed, but it's from a different farm and as such it is priced differently.
Even the timing for when you buy green coffee is important. I bought a pallet of a bean (a ton) and then the same lot number was 20% more expensive on the next pallet the following month because I did not forward contract.
Most of the suppliers who offer green beans to home roasters range very good (even go so far as to say excellent) grade coffees. These can be unique, high quality beans from small farms or microlots.
If you have ever tried to open jute bags (sometimes it is very fiddly and frustrating), scoop and weigh beans into a package, then seal, label, prepare the box, label, ship, etc. then you might realize this is a VERY time consuming and labor intensive activity. This labor effort needs to be accounted on top of the high grade beans you are purchasing.
In my mind, you are getting a damn good deal when you purchase green beans from the above mentioned providers - I still don't know how they do it for the price actually.