I am in Paul's camp on this one and I also find Moses and Frederick proffer salient points too.
Provenance over price has its place and describes how I have put my own collection together. I would never deny that I am enthralled by finding a true Port gem at a bargain price, once assured of the provenance. But, I'd rather spend an extra $20 on a bottle to guarantee that 10 years down the road or more, when popping the bottle open for a very special tasting or a celebration with friends/family ... I want to limit to the greatest degree possible ... the disappointment that can come from a bottle purchased from a less than perfect environment. Money then, becomes inconsequential.
Whether purchasing on first tranch en primeur or "pre-arrival" as I begain doing with the 1991 vintage, (I believe this practice only began in the USA with the 1985 vintage ... an interesting tidbit of trivia) there have been vintages like 1994 where I was thrilled I had locked and loaded, early and often. Not only did the prices escalate rather quickly compared to some of the older vintages around at that time for what seems today like stellar pricing. Nonetheless, please don't be fooled and think that only Nacional has been worth purchasing as futures.
In fact, it was not really until the "campaign" of the 2000 Vintage, which was offered as futures (Port, not Bdx!

) that any vintage that I've seen sold as a pre-release or en primeur, wound up being offered below that price after the VPs were released in No. America. I am sure there may have been others in earlier market cycles of the Port trade that took place in the UK market, but not here in the USA. This set precedent and also set off a terrible turn of events in my opinion, which is the reason why today there is so much skepticism amongst Port enthusiasts when it comes to futures. It is not for everybody and
there is "risk" at stake.
It is not a game to be played, unless thoroughly understood in advance.
As Moses mentioned, there are not going to be
any $70 Vintage Ports from a vintage like 2005 that I would recommend in terms of value for money. That would just be a silly expenditure in my opinion. Although 2005 is a solid vintage, it is a far cry from the quality (in the vast majority of cases) offered by the likes of the 2003 and 2000 Vintage Ports. Those two are just to mentioned, to supply comparative perspective. But as Moses said, for that kind of money you can find quality Ports with 2 decades of bottle age and the carrying cost absorbed, usually by a retailer/agent.
That is why when I recommend the 2004 or 2005 Vintage Ports, I make sure to put them in perspective and realize they are not "classic" better known as "generally declared vintages" which seem to have typically maintained their value and seen it increase very differently than the in-between vintages. Therefore, when assessing purchases of an "off vintage" for your near or mid-term drinking pleasure, the quality of Port must also be guaged by its selling price and the inherent value that represents compared to the classics, but also to the Ports that have already aged in the bottle.
Buying on futures is a gamble, in terms of how the price of your bottle or case will escalate or decrease over time. As Paul mentioned though, it really boils down to whether or not you are looking to put away pristine bottles in your cellar and keep them there for the years/decades required to drink them when mature and/or at peak. Or viewing through another viewpoint, would you rather purchase the same wine at a later time paying either more or less, but usually not really sure where the Port has spent its time. If you like to gamble and are not adverse to risk, please by all means do continue to source your Ports from auction.
Just my two cents.