As mentioned above, market buying index tokens incurs no fees. However, when buying large amounts (e.g > $10,000), slippage costs might be significant. Therefore, you might be better off minting the liquidity pool (LP) tokens corresponding to the desired index. Minting does not incur swap fees if you provide all the underlying pool tokens in the weights suggested by the UI, but does if you provide only one of the underlying tokens, or mint via the router using ETH, DAI or USDC. The two latter options can be quite gas intensive, and should only be considered for buys greater than US$10,000.