I have recently flown on SilkAir from Singapore to Phuket and could not justify paying more than five times more (S$1,300) for a 90-minute flight - the same flight in Economy costs approximately S$240. With that being said, the MI Upgrade is something that people do not talk about often. It is basically a bidding system that allows you to make an offer to SilkAir to upgrade your flight from Economy to Business - if the bid is unsuccessful, you do not pay anything at all and if it is, you pay the amount that you have offered.
While the fare class that I was booked into from Singapore to Phuket did not allow for me to submit a bid, it was available on the return flight (HKT-SIN). Remember that if your offer gets accepted by SilkAir, your seat will be upgraded from Economy to Business and you will get to enjoy benefits like pre-departure lounge access, better food, an enhanced 40kg baggage allowance (up from 30kg in Economy) as well as other hard-product benefits. With that being said, your mileage accrual rate would be as per your original booking class - you will also not be earning any PPS Value with an upgraded flight.
I have not been targeted for any bidding upgrades on my recent flights on Singapore Airlines and SilkAir but most of these flights have been on award redemption and therefore, it would not make sense to target me at all. However, you can always check whether your upcoming flights are eligible for an upgrade bid by heading down to the MI Upgrade page. The minimum bid from HKT to SIN for me was S$120 and the maximum was capped at S$300. Unless you are really considering an upgrade from Economy to Business, the thing that most travellers do is to submit the minimum bid - essentially offering SilkAir S$120 to upgrade you from Economy to Business. Obviously, submitting the lowest bid would guarantee the lowest probability of upgrade amongst the rest but hey, you really are just trying your luck here.