I think you're overlooking things. Bonds are a really good investment at the moment. Currently gp is expensive if u buy off jagex, but in the longer term, as more people choose to buy members with gp by purchasing bonds, the price of bonds will rise to a level where buying gold off the black market/jagex is indifferent in cost for the buyer.
What one can do now is, as an alternative to Option 1(buying mems off jagex) is Option 2: Buy GP off black market for .32/mil> Buy 2 bonds for 6M each> Cash in bonds for 1 months mems. Total cost for 1 months mems: 12*$0.32~$4 bux.
Alot of players will do this as it's a cheaper way of getting membership... Thus causing the demand for bonds to rise (at current prices). This will cause the blackmarket price of GP to increase (as more ppl are buying GP off the black market to purchase bonds to exchange for mems) and the bond price to increase. Equilibrium will be reached when the cost of obtaining members via bonds is no different (or maybe a bit less) to the cost of buying it in the regular way from jagex (8 bux?)...
If the price of blackmarket GP rises to say $0.35/mil, then we can calculate the corresponding bond price: $8/$0.35 = ~23m/2 = 11.5m. (divide by 2 as it takes 2 bonds for 1 months membership). This is the bond price that would make players indifferent from buying mems the regular way (buying from jagex for $8) or buying mems using Option 2: buy rsgp from blackmarket>exchange gp for bonds>exchange bonds for membership.
*Because the 2nd option is a hassle, it makes sense that it will give some sort of money saving for the effort, e.g. saving of a dollar or 2. If people deemed the effort worth $1 saved per month of membership, then the equilibrium bond price that would make ppl indifferent from the 1st or 2nd option of obtaining members would be: $7/$0.35=~20m/2=10m each... etc.
...The price of bonds will rise as people see them as a cheap alternative to purchasing mems (through use of blackmarkets)... They will stop rising in price when the money saved purchasing members using Option 2 is deemed insufficient by the market for the additional effort it requires.
I say before too long bonds will rise to and stabilize at ~12m per. This would equate to GP costing: $5/12=$0.42 cents per mil off jagex, which is only marginally more than black market prices.
If this causes black market prices to decrease in order to be competitive against jagex, then this will have the effect of people using Option 2 to get membership again... as people do this the bond price will rise again. E.g. if GP falls to $0.2 in black markets, then referring back to *, the price of bonds will need to be: $7/$0.2=35m/2=17.5m per in order for players to be indifferent from obtaining mems via Option 1 or Option 2.... As the bond price increases, buying GP from jagex in this example would now be: $5/17.5=$0.286 which is close to the black market price of $0.2/m....(The cost of GP from jagex moves with the blackmarket, keeping the gap small). We can conclude that goldsellers lowering prices will not be beneficial to them as the price of GP from jagex will fall as well. As the cost of GP from Jagex is only slightly higher than the cost of GP from blackmarkets, blackmarkets will lose business.
Option 2 only exists if blackmarkets do... so blackmarkets will continue to operate, but with less business. Business will decline greatly.
All this analysis is based on the assumption that RS players are rational and see Option 2 as a viable way of obtaining members, which many of them won't. Hence it is flawed but you get the idea..
