Costs are lower than in South Africa. Food and public transport are slightly less expensive, while imported items like film are more expensive and only reliably available in Maseru. Pony trekking costs about M120 a day at most of the lodges. A ten percent sales tax is applicable, and often not added to marked prices on menus or in supermarkets and shops.
Most major credit cards , such as Visa, Mastercard and American Express, are of limited use. Hotels and lodges geared to foreign visitors accept them, as do a few Maseru restaurants and the craft shops of Teya-Teyaneng. Otherwise, you need cash for everything.
Lesotho's currency is the maloti (M), divided into 100 lisenti, which is tied to the South African rand (R1 = M1). You can use rand in Lesotho, so you don't need to convert them beforehand, but you cannot use or exchange maloti anywhere outside Lesotho, so make sure you use them up or exchange them before leaving.
There's no need to convert South African rand. Banks are found in all Lesotho's towns, but Maseru is the only place where it's easy to change money at them. Hours are Mon-Fri 8.30am-3.30pm, closing 1pm on Wed, Sat 8.30am-11am (avoid busy Saturdays). While a few ATMs exist, not all accept foreign cashcards. The larger hotels in Maseru and the lowland towns change money, but for ludicrously bad rates, making them very much a last resort.