If we assume that each of the mixers is as capable as your existing one, you'd have to consider how much it saves you, and compare that to the cost of the new mixer.
New, that model of mixer when new goes for US$10,000 to $15,000. Accessories will increase the cost; the scraper blade you mentioned runs around $500.
We have two ways to calculate the value to you; we can base it off of time savings of using the new mixer, but those calculations can be a bit messy. If you're planning on buying this to grow into it, and only going to be doubling recipes, your time savings from mixing may be lost from cleaning (if it doesn't fit in your sink), or just hauling it around (shipping weight for a replacement bowl is listed as 47lbs/21kg); even if the bowl is only half that, a full batch might be 60lbs or more).
If you value your time at $8/hr, at $11,0000, you'd need 1375 hours to start seeing a benefit. If you only saved 1hr per day, working 5 days per week, this would take over 4 years to start seeing a benefit. If the cost with accessories is near $16,000, you'd need 2000 hrs, or almost 6 years).
These time savings may seem low, but remember that you're not necessarily quadrupling your batches in the same time; you still have to consider time to scoop them out, bake them, etc. If you don't have sufficient oven & cooling space, you may not be able to take full advantage of the larger mixer.
You may value your time as more than $8/hr, and that would improve your cost recovery time ... but also consider that if you value your time highly, you could instead hire extra help. If you hired someone half time at $15/hr, it would take 10.5 to 15.25 months to recover the cost.
Your second alternative for calculations is in how much additional profits you could make. So, if you could turn out an extra 400 cookies per day (and could actually manage to sell them all), you'd consider the cost recovery time to be the increased profits minus the increased costs (ingredients, extra sales staff, etc.)
And for both of these calculations when we're looking at long time-scales, you have to consider what they call the 'time value of money' ... or what your effective interest rate is. If you have the money saved up, it may be near 1%, or whatever you can invest it at (with low risk). If you have to borrow the money, it may be 15% or more if you're putting it on a credit card. It should be somewhere in between with a bank loan.
So, that 6 year 'payback' value with a savings of 1 hr per day at $8/hr, if you had a 15% interest will actually take 27 years to pay off (calculator), as you only make back $140/month. If you were to save 1.25hrs/day ($160/month), we cut the time down to 13 years.
So, to summarize, some of the considerations:
- can you fit the bowl in your sink to clean?
- will you be able to manage the bowl when it's filled? (may be able to get a dolly to move it around)
- how expensive are all of the accessories you need?
- would you be able to take advantage of the size, or should you consider a smaller (20qt, 30qt mixer)?
- how much time will this save you per day/week/etc?
- how much extra profit could you make if you had this?
- are there other things you could do to expand other than the mixer? (eg, hire extra help)
- how long would it take to recover the cost of the mixer?