The only reason there is a minimum reasonable balance to use a SimpleAllocation plans is due to the fact that your returns are reduced by any trading fees and/or loads (only applicable for mutual funds) you incur when rebalancing your account. If you have no trading fees (like in a 401K for which all fund have no fees/loads), then there really is no minimum balance required.
If you do incur transaction costs when rebalancing, then there is not an exact answer to this question. The trading fees will reduce your returns. So the minimum balance you should have in order to use a SimpleAllocation plan depends on: the amount your broker charges to make a trade, your expected return, and how much of the expected return you are willing to pay in trading fees.
Here is an example. Lets say your broker charges $10/trade. (Please make sure you know the commission rate for your broker, and adjust the example below by the actual commission you will pay.) At the time this is being written, SimpleAllocation plans will allocate to a maximum of 6 securities at one time. For a plan that reallocates each month, that means that the absolute maximum number of trades per month would be 12, or 144 trades/year. (That is an absolute worst case, as it assumes that all 6 securities you are in need sold, and a different 6 securities purchased, every month of the year. The ETF_Group_001 has averaged 95 trade/year for years 2002-2012, while ETF_Group_002 has averaged about 75 trades/year over the same period.) The maximum trading fees are then 12 * $10 = $120, or $1440/year.
If your total account balance were $14,400, and you were hoping to achieve a 10% annual return ($14,400 * 10% annual return = $1,440 annual profit), SimpleAllocation plans are not for you, because your entire profit would be spent in brokerage fees. If your account balance were $1,440,000, the trading fees represent only 1% of your expected return ($1,440,000 * 10% annual return = $144,000 annual profit, $1,440 / $144,000 = 1.0% of return paid in fees).
If the allocation plan you choose has mutual funds, you may also be subject to additional fees that you will need to account for. Pay particular attention to additional fees that result from "early" redemption; this is a fee that is frequently charged if the security is held less than 90 days.
Prior to using any SimpleAllocation plans please find out the commission rate for your broker, go through the example above using your actual commission rate, understand the maximum amount of fees to which you will be subject, including mutual fund fees, and make sure the fees are reasonable given the returns you hope to achieve.