does anyone know any US small cap or mid cap trackers that are low cost?
Maybe the SPDR S&P 400 Mid Cap tracker?
It's an ETF (ticker SPY4) with a TER of 0.3%.
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does anyone know any US small cap or mid cap trackers that are low cost?
It the fund fee is low then there isn't much in it cost wise between a fund and an ETF. As long as you don't trade them often (you buy and hold) generally an ETF will be cheaper than a mutual fund but if you want to do regular investments and reinvest dividends easily then a low fee fund might be better.
Look for anything with SPDR in its name to find ETFs that track US indexes. You can just buy and sell these like shares.
If its a fund you want then an index tracking fund will have fees of about 0.10% from most brokers. I have L&G US Index Acc which has that low fee but isn't specifically aimed at Mid-Caps.