If the ME3600X is much too expensive, it's possible your expectations for pricing aren't realistic.
Selective QinQ is pretty new, and tends to be found only in provider type equipment. If you can live without that, "normal" (non selective) QinQ is offered on most switches today. In that case several different Extreme switches might be of interest.
Steinar Haug, Nethelp consulting, sthaug@nethelp.no
Finding switches that will do private vlans, selective QinQ and the requirement for 24SFP ports is a combination that adds up to a switch in the $5,000 to $7,000 range even without layer 3 from most of the vendors whose products I use. Best one I have been able to come up with price-wise that seems to meet your requirements is Huawei Quidway S5300 (S5328C-EI-24S) which seems to be in the $3000 neighborhood but I have never used their gear.