Some details in case you are interested:
Being a defined benefit kind of pension plan, the formula for your Social Security benefits isn't tied directly to FICA contributions, and I'm not aware of any calculator that performs an ROI based on FICA contributions.
Rather, how much you'll get in retirement is based on your average indexed monthly earnings. Here's some information on the Social Security calculation from the Social Security Administration - Primary Insurance Amount (PIA):
For an individual who first becomes eligible for old-age insurance
benefits or disability insurance benefits in 2013, or who dies in 2013
before becoming eligible for benefits, his/her PIA will be the sum of:
(a) 90 percent of the first $791 of his/her average indexed monthly earnings, plus
(b) 32 percent of his/her average indexed monthly earnings over $791 and through $4,768, plus
(c) 15 percent of his/her average indexed monthly earnings over $4,768.
Here's an example. Of course, to calculate a benefit in the future, you'll need to calculate projected average indexed monthly earnings; more details here. You'll also need to make assumptions about what those bend points might be in the future. The average wage indexing values for calculating the AIME are available from the Social Security Administration's site, but future indexing values will also need to be projected based on an assumption about their inflation. You'll also need to project the Contribution and Benefit Base which limits the earnings used to calculate contributions and benefits.
Also, the PIA calculation assumes benefits are taken at the normal retirement age. Calculating an early or late retirement factor is required to adjust benefits for another age. Then, whatever benefits you get will increase each year, because the benefit is increased based on annual changes in the cost of living.
Performing the series of calculations by hand isn't my idea of fun, but implementing it as a spreadsheet (or a web page) and adding in some "ROI based on FICA contributions" calculations might be an interesting exercise if you are so inclined?
For completeness sake, I'll mention that the SSA also provides source code for a Social Security Benefit Calculator.