What makes a LEGO Investment Viable?

When LEGO investors evaluate a given set, they ask themselves, is this investment viable? Is it worth putting my hard earned money into? But, what does viable mean exactly? Is it anything that nets more than the inflation rate or the supposed risk free investment rate of US treasuries (let's not debate if treasuries are truly risk free)? Probably not. We all know there is a certain element of risk involved in LEGO investing that should command some risk premium, but what should that risk premium be? Fcbarcelona101 points out in his very insightful blog, that even among various LEGO themes, risk can vary greatly.

Other questions may be, Can I flip this set today for a quick profit or should I hold on until EOL or hold until the community standard of EOL + 2 Yrs. What time frame should I expect to realize a return on my investment? What level of growth should I expect from a given set? If I have a limited budget and a high tolerance for risk, should go for that one home run? What are my goals? The answers to these questions and ultimately the question of what makes an investment worthwhile are going to be different for everyone. There a lot of factors involved. Many investors prefer to deal in large sets which can net $100+ in one sale while others are content to deal in volume and sell polybag after polybag for a net of $3 each. We all have to evaluate our budget, storage space, and the amount of time we want to devote to this hobby/job/casual pastime among other things. Many of these factors are difficult or impossible to quantify.

However, in an effect to quantify what can be quantified, I put together a calculation in the attached spreadsheet that encompasses the most relevant variables that I consider when deciding to purchase a given set. It is not meant to be all inclusive, for example it doesn't include a risk factor which I think would be a very nice addition. Here it is, perhaps it will be useful to some:

The fields which are intended for the user to populate are highlighted in yellow:
MSRP - Enter price paid
Hold Time (Yrs)
Time Value of Money Discount Rt - Enter the annual return you expect
Transaction Cost - Enter the % of the final sale price lost in transaction costs
Sales Effort - time involved in the investment (buying, listing, selling, shipping) entered as fraction of an hour
Hourly Rate - How much is your time worth to you or the amount of money per hour you would need to make if your transaction were to net '0' and have it still be worthwhile.

The resulting output gives:
Future Value:
Net Return
Worthwhile?